Symposium Organizers
Ting He ConocoPhillips
Karen Swider-Lyons Naval Research Laboratory
Byungwoo Park Seoul National University
Paul A. Kohl Georgia Institute of Technology
Harry L. Tuller Massachusetts Institute of Technology
GG1: Polymer-electrolyte Fuel Cells I
Session Chairs
Monday PM, November 29, 2010
Back Bay A (Sheraton)
9:30 AM - GG1.1
Role of Confinement on Swellability, Solubility and Diffusivity of Nafion Thin Films.
Scott Eastman 1 , Shuhui Kang 1 , Christopher Soles 1 , Kirt Page 1
1 Polymers Division, NIST, Gaithersburg, Maryland, United States
Show AbstractThe absorption, desorption, and diffusion of water in Nafion is extremely complex, with literature values for water diffusivity varying over several orders magnitude in some cases. In part, this variation can be attributed to inconsistencies in sample preparation and handling. Changes in water absorption/desorption and diffusion in Nafion membranes can be affected by membrane thickness, annealing conditions, processing methodologies, and water vapor concentration. While these relationships have been studied in great detail for bulk membranes, there has been very little work done on Nafion thin films. It has been well documented that polymer thin films (typically <100nm thick) demonstrate significant material property deviations from bulk values. These deviations are believed to be mediated by interactions at each polymer interface. Understanding these interactions is vital to devising proper models for more complex systems such as fuel cells, which consist of several polymer interfacial interactions, for example, at the triple phase boundary of a membrane-electrode assembly (MEA). These studies will also be beneficial in understanding the behaviors of composite membranes as more complex membrane systems are being realized with the incorporation of various fillers and additives. The work presented here focuses on studying the water absorption and diffusion in Nafion thin films as a function of film thickness (l), keeping other variables consistent. Utilizing specular x-ray reflectivity (SXR) we have measured the extent of swelling and the equilibrium water content as a function of relative humidity and film thickness for films less than 220 nm. We observe that the equilibrium swelling and water content is constant at a set relative humidity for films above 100nm. Films thinner than 100 nm show suppression in equilibrium swelling and water content with decreasing film thickness and approach 65% suppression in both quantities for 20 nm films. To complement the swelling studies, the diffusion of water vapor through the thin film membranes has been studied by employing polarization-modulation infrared reflection-absorption spectroscopy (PMIRRAS). By modeling the growth of the water peak in the IR spectrum as a function of time, we have determined the diffusion coefficient (D) as a function of film thickness. These results show that, along with suppression in swelling, there is a power-law relationship between D and l. More specifically, we find that films in a thickness range of 20 nm to 220nm reveal a diffusion coefficient that is 4-5 orders of magnitude slower than that measured in bulk membranes. Understanding water absorption, desorption, and diffusion of thin Nafion films gives us valuable insight into the interfacial interactions experienced in a working fuel cell and is vital to the development and optimization of Nafion membrane interfaces with respect to water management and conductivity.
9:45 AM - GG1.2
Preparation and Characterization of Multi-monomer Pre-irradiation Grafted Proton Conducting Membranes for Fuel Cells.
Lorenz Gubler 1 , Frank Wallasch 1 2 , Kaewta Jetsrisuparb 1 , Hicham Ben youcef 1 , Guenther Scherer 1
1 Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen PSI Switzerland, 2 , present address: Across Barriers GmbH, Science Park 1, 66123 Saarbruecken Germany
Show AbstractThe main challenges for the introduction of the polymer electrolyte fuel cell (PEFC) technology in various applications, for instance electric cars, are high cost and insufficient lifetime under application-relevant conditions. Radiation grafted membranes based on sulfonated polystyrene or derivatives thereof offer a potentially cost-effective alternative to commonly used perfluoroalkylsulfonic acid (PFSA) membranes. The preparation involves the electron-beam irradiation of a fluorinated or partially fluorinated base film. A graft copolymer is subsequently formed by introducing the irradiatied film into a monomer solution. Proton conductivity is obtained by sulfonation of the grafted film. The chemical aging of the proton exchange membrane (PEM) by radical intermediates, such as HO*, limits the durability of the fuel cell. Crosslinked radiation grafted membranes show a substantially improved durability. Yet it is of interest to understand the underlying radical attack mechanisms in poly(styrene sulfonic acid) (PSSA) based membranes and identify means to improve the intrinsic chemical stability of the material. We have shown that favorable combinations of grafting monomers, such as alpha-methylstyrene (AMS) or styrene and methacrylonitrile (MAN), yield membranes with enhanced durability. In this contribution, the implications of using multi-monomer grafting systems are discussed: the co-grafting of styrene and MAN, for instance, requires an understanding of the co-polymerization kinetics. Also, relevant characterization techniques have to be adopted to quantify film composition and modification during processing. Furthermore, fuel cell relevant membrane properties, such as ion exchange capacity, water uptake and proton conductivity, are presented. In situ characterization of membranes in the fuel cell show the improved chemical stability of AMS / MAN and styrene / MAN co-grafted membranes over pure styrene grafted membranes. Potential stabilizing mechanisms will be discussed.
10:00 AM - **GG1.3
High Temperature Proton Conducting Polymer Membrane: Studies with the PBI/Phosphoric Acid System.
Robert Savinell 1 , Tyler Petek 1
1 Chemical Engineering, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractA proton conducting polymer material operating at elevated temperatures under dry conditions has been of interest in recent years for application in PEM fuel cells. Operating fuel cells at elevated temperatures offers advantages in extending fuel impurity tolerance and in system simplification since humidification is not as much of an issue as in typical perfluorinated sulfonic acid proton conducting polymers. Of all the approaches to achieving a high temperature proton conducting polymer membrane, PBI/phosphoric acid has shown significant promise. In this presentation we will summarize some of the recent models, calculations, and experiments we have performed of the transport processes in the PBI/PA system leading towards a better understanding of the durability of this system especially under conditions of high current densities. We have been investigating PBI/PA membranes in electrolyzers to purify and pressurize hydrogen and will summarize our experimental hydrogen pump data with a PBI/PA membrane electrode assembly. 1Qingfeng Li, Jens Oluf Jennsen, Robert F. Savinell, and Niels J. Bjerrum, Progress in Polymer Science, 34(5), 449-477(2009)
10:30 AM - **GG1.4
Membrane Electrode Assembly for Polymer Electrolyte Membrane Fuel Cells.
Taegeun Noh 1 , Seongho Choi 1 , Sangwoo Lee 1 , Hyuk Kim 1
1 Corporate R&D, LG Chem Research Park, Daejeon Korea (the Republic of)
Show AbstractPolymer Electrolyte Membrane Fuel Cell (PEMFC) is the most promising power source for next generation transportation and residential applications. For mass commercialization of PEMFC system, one of the main barriers is the high cost of Membrane Electrode Assembly (MEA) using expensive fluoropolymer membrane and Pt catalyst. LG Chem has been developing MEA with hydrocarbon based membrane and ink-jet coating technology. Hydrocarbon membrane has advantages of low price and high thermal stability. Our proprietary multiblock copolymer, which showed high proton conductivity, was improved in water management by morphology control and the hydrocarbon based membrane was optimized for MEA application. Recently, its comparable performance and chemical stability to those of common fluoropolymer membranes increased the potential for an alternative membrane. Ink-jet is a non-impact dot printing technology in which small droplets of ink are jetted from nozzle plate to substrates such as papers and polymers. Drop-on-demand of Pt catalyst ink using ink-jet reduces material loss significantly and enables electrode patterning to open the way of designing the next generation MEA. Using Pt catalyst inks specially formulated for ink-jetting and ink-jet systems with modified print heads, continuous and homogeneous jetting was achieved and successfully applied to MEA electrode manufacture. It was found the ink-jet electrode had very flat and homogenous layer with few crack and played a role in increasing performance. In the presentation, the improvement and further development of materials and MEAs at LG Chem will be discussed.
11:30 AM - **GG1.5
Design and Synthesis of Advanced Nanoscale Electrocatlysts.
Chao Wang 1 , Dusan Strmcnik 1 , Dusan Tripkovic 1 , Nenad Markovic 1 , Vojislav Stamenkovic 1
1 , Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractAbility to tune the electronic and structural properties of nanocatalysts can potentially lead towards the superior catalytic enhancement that was reported for the Pt3Ni(111)-skin surface [1]. Fine tuning of the surface properties is usually done on extended well-defined surfaces in ultra-high vacuum. A number of surface sensitive tools could be utilized such as AES, LEIS and UPS before controlled transfer into real reaction environment. The single and polycrystalline crystalline well-defined surfaces have been used to benchmark the activity range that could be expected on Pt based electrodes. The knowledge accumulated from well-defined systems is further used to engineer nanoscale surfaces with designated composition and morphology. It has been proposed that surface modifications induced by the second/third metal, and consequent catalytic enhancements could occur through the following effects: (1) Electronic effect, due to changes in the metallic d-band center position vs. Fermi level; and (2) Structural effect, which reflects relationship between atomic geometry, and/or surface chemistry, i.e., dissolution – surface roughening. In principle, different near-surface composition profiles have been found to have different electronic structures. Modification in Pt electronic properties alters adsorption/catalytic properties of corresponding materials. The most active systems for the electrochemical oxygen reduction reaction (ORR) are established to be the Pt skin near surface formationThe similar levels of catalytic enhancement have been established for corresponding nanoscale materials. In addition to electronic properties we have found how catalytic activity could be affected by the concentration profiles of nanoscale surfaces. Ability to control surface and near surface catalyst properties enables fine tuning of catalytic activity and their durability.[1] V. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. Markovic, Science 315 (2007) 493-497.[2] V. Stamenkovic, B.S. Mun, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, J. Rossmeisl, J. Greeley, J.K. Nørskov, Angewandte Chemie International Edition 45 (2006) 2897.[3] V. Stamenkovic, B.S. Mun, M. Arenz, K.J.J. Mayrhofer, C. A. Lucas, G. Wang, P.N. Ross, N.M. Markovic, Nature Materials 6 (2007) 241.
12:00 PM - GG1.6
Operation of PEM Fuel Cells in Contaminated Environments.
Karen Swider-Lyons 1 , Olga Baturina 1 , Benjamin Gould 1 , Yannick Garsany 1 2
1 , Naval Research Laboratory, Washington, District of Columbia, United States, 2 , EXCET Inc., Springfield, Virginia, United States
Show AbstractProton exchange membrane fuel cells (PEMFCs) are susceptive to loss of performance due to poisoning by contaminants in the environment and from the fuel cell system. Performance loss can be caused by the decrease in catalytic activity through the adsorption of contaminants on the catalyst surface, or due to the loss of conductivity in the H+-Nafion membrane due to exchange with impurity cations. We are studying three aggressive poisons: airborne sulfur species (SO2, H2S and COS), sodium chloride (NaCl), and the glycol-based liquids used as the system coolant and how they poison the materials in the PEMFC. By understanding the extent and process of the poisoning, we have developed electrochemical methods based on voltage cycling to regain the performance of contaminated fuel cells. The methods are materials specific. For instance certain catalysts, such as Pt-alloys, recover their activity at lower voltages than pure Pt catalysts, which we explain in terms of how they adsorb oxygen. Voltage cycling is typically necessary to release the charge on the Pt catalysts, and desorb charged species from their surface.
12:15 PM - GG1.7
Coating Vulcan Carbon Electrocatalyst Support with Tantalum Oxy-phosphate Film Boosts Pt ORR Activity and Durability.
Albert Epshteyn 1 , Yannick Garsany 1 2 , Karren More 3 , Andrew Purdy 1 , Karen Swider-Lyons 1
1 , Naval Research Laboratory, Washington, District of Columbia, United States, 2 , EXCET INC., Springfield, Virginia, United States, 3 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractA low density monolayer tantalum oxy-phosphate gel coating immobilizes platinum nanoparticles on carbon supports. When the resulting nanocomposite is heated under a 10% H2 atmosphere in N2 up to 500 °C, there is no observed change in the crystallite size from the Scherer analysis of the XRD, while at higher temperatures particle ripening can be used advantageously to tune the average Pt particle size, and in turn the electrocatalytic activity. We have shown the utility of this heterogeneous catalyst support system as it produces high mass activity (0.46 A/mg Pt) while also having significantly improved stability (durability) in the harsh environment of the oxygen reduction reaction (ORR) in PEM fuel cells.
12:30 PM - **GG1.8
Supported Catalyst for PEFC.
Chanho Pak 1
1 Fuel Cell Group, Energy Lab, Samsung Advanced Institute of Technology, Samsung Electronics, Yongin-si Korea (the Republic of)
Show AbstractPolymer electrolyte fuel cell (PEFC) is defined by the membrane electrolyte based on the polymer membrane. Nowadays, PEFC can divide into the polymer electrolyte membrane fuel cell (PEMFC) and direct methanol fuel cell (DFMC) by the fuel such as hydrogen and methanol, respectively. These fuel cells are about to commercialize for the residential and portable power applications according to the decent efforts from the academia, governments and industry. However, there are still challenges about the durability and cost of the fuel cells for expanding the market size. In a view of the cost, the Pt used mainly as catalyst is considered as a most expensive component and should be reduced by increasing the activity of catalyst or introducing cheaper materials. There are two main approaches to increase the catalyst activity. One is the addition of another metal to form an alloy with Pt and the other is the increase the utilization of Pt nanoparticle by using a novel support. For replacing the Pt, the cheaper non-pt elements like Pd and Ru have been extensively developed as a possible catalyst for PEFC. Recently, in our lab, the novel support for the highly loaded Pt catalyst has been synthesized and applied to the membrane electrode assembly (MEA) for the DMFC. In this presentation, the author would like to introduce the method and development route of ordered mesoporous carbon as novel support and demonstrate the mitigation of Pt amount in the MEA. And then, the author would like to present the very recent effort for removing the Pt in the MEA by non-Pt catalyst for anode and cathode for the intermediate temperature PEMFC, which used a polymer membrane with phosphoric acid at 150 oC.
GG2: Polymer-electrolyte Fuel Cells II
Session Chairs
Monday PM, November 29, 2010
Back Bay A (Sheraton)
2:30 PM - GG2.1
Nanoengineered Pt-based Multimetallic Cathode Electrocatalyst in Proton Exchange Membrane Fuel Cells.
Bin Fang 1 , Jin Luo 1 , Bridgid Wanjala 1 , Rameshwori Loukrakpam 1 , Xiang Hu 1 , Jordan Last 1 , Chuan-Jian Zhong 1
1 Department of Chemistry , State University of New York at Binghamton, Binghamton, New York, United States
Show AbstractWhile the characterization of electrocatalytic activity of nanostructured electrocatalysts for fuel cell cathode reaction (i.e., oxygen reduction reaction (ORR)) were mostly based on rotating disc electrode (RDE) measurements, the detailed evaluation of such catalysts by a combination of both RDE and real fuel cell measurements provide valuable fundamental insights into the assessment of the nanostructured activity and stability of the catalysts, The presentation describes the findings of such an investigation of a series of nanoengineered Pt based multimetallic cathode electrocatalyst in proton exchange membrane fuel cells (PEMFCs). Examples of the discussion will focus on one bimetallic catalyst, gold-platinum (AuPt) nanoparticle supported on carbon, and one trimetallic catalyst, platinum-vanadium-iron (PtVFe) nanoparticle supported on carbon. The membrane electrode assembly was prepared using carbon-supported AunPt100-n or PtnVmFe100-n-m nanoparticles with controlled sizes and compositions that were thermally treated under controlled temperature, atmosphere, and time. The electrocatalytic performances of these catalysts in the fuel cells was found to be dependent on the composition and the nanoscale phase properties which are controlled by the thermal treatment parameters. For AuPt/C catalysts, excellent fuel cell performance was observed for the catalysts which are characteristic of an alloyed AuPt phase with a lattice parameter approaching that for an Pt-rich alloy phase. The observed combination of high activity and high durability of the selected AuPt catalysts indicated that the nanoengineered bimetallic/trimetallic catalyst systems, upon further refinement and optimization of the nanoscale phase properties and durability serve as a promising candidate of electrocatalysts for the practical application in fuel cells.
2:45 PM - GG2.2
Facile Preparation of Carbon Supported Co-Pd Alloy and Core-shell Nanoparticles by Ultrasound and Their Enhanced Electrocatalytic ORR Activity.
Ji-Hoon Jang 1 , Min-Hye Kim 2 , Yong-Tae Kim 1 , Jeong-Gyu Park 1 , Young-Uk Kwon 1 2
1 Chemistry, Sungkyunkwan university, Suwon Korea (the Republic of), 2 Center for Human Interface Nanotechnology, Sungkyunkwan university, Suwon Korea (the Republic of)
Show AbstractCarbon supported Co-Pd nanocomposites with different surface structure and various ratio of Co/Pd were synthesized by ultrasound assisted polyol process. Sonochemical treatment of palladium acetylacetonate and cobalt acetylacetonate in ethylene glycol in the presence of a carbon support without any other surfactant, reagent for pH adjustment or stabilizer produced 4-7 nm sized CoxPd nanoparticles dispersed on the carbon support. The absence of additional reagents in the sonochemical reaction system makes unsoiled nanoparticles in comparison with conventional wetting methods. The structures of the nanoparticles, pure Pd, Co-Pd alloy and core-shell structures were characterized by XRD and TEM. Electrocatalytic oxygen reduction reaction (ORR) behavior of the materials was measured by rotating disk electrode (RDE) method and compared with pure Pd/C and commercial Pt/C. We obtained a larger mass activity for ORR with Co-Pd mixed phase samples than pure Pd/C and a comparable result with commercial Pt/C. Especially, Co0.25Pd core-shell structure shows a remarkable enhanced activity for ORR.
3:00 PM - GG2.3
Rational Synthesis of Pd-TM/FePt (TM = Co,Ni,Cu) Core/Shell Nanoparticles for Catalytic Oxygen Reduction Reaction.
Vismadeb Mazumder 1 , Shouheng Sun 1
1 Chemistry, Brown University, Providence, Rhode Island, United States
Show AbstractThe need to limit precious metal usage in catalysis has lead to numerous syntheses of core-shell nanostructures that increase catalytic efficiency via decreased loading. Here we report a generalized seed-mediated approach towards creation of multi-metallic core/shell nanoparticles (NPs) with less than 10% Pt in the sub-10 nm size range. First, a surfactant mediated process produced composition and size controlled Pd-TM alloy nanoparticles (NPs). These NPs were used as seeds to grow a 1nm Pt-containing shell, resulting in the desired composition controlled core/shell nanostructures. The mono-dispersity of the seeds was critical to the accurate construction of the shells. These surfactant coated core-shell NPs were further supported on the Ketjen carbon, and were found to be readily “cleaned” by a 99% acetic acid wash. To demonstrate the impact of the core-shell interfacial interaction in fuel cell electrocatalysis, these core-shell/C NPs were evaluated for the Oxygen Reduction Reaction (ORR) in 0.1M Perchloric Acid at 308K. The catalysts show unprecedented core component dependent ORR activity and durability. This study opens up a new vista to exploit core-shell interactions for future development of multi-component NPs with unique properties for catalytic applications. Reference: 1.V.Mazumder, M.Chi, K.L. More, S.Sun. J. Am. Chem. Soc., 2010, 132, 7848.
3:15 PM - GG2.4
Characterization of Lattice Structural Effect of Trimetallic Alloy Nanoparticles on Electrocatalytic Activity of Oxygen Reduction Reaction.
Bridgid Wanjala 1 , Rameshwori Loukrakpam 1 , Bin Fang 1 , Jin Luo 1 , Chuan-Jian Zhong 1
1 Department of Chemistry , State University of New York at Binghamton, Binghamton, New York, United States
Show AbstractFuel cells are attractive power sources with high conversion efficiencies and low pollution. The high cost, low activity and poor stability of many existing Pt-based cathode catalysts for oxygen reduction reaction (ORR) constitute a major area of problems in the commercialization of fuel cells. This presentation reports our recent findings on the correlation between the lattice structural properties and the electrocatalytic performance for a series of nanostructured trimetallic nanoparticle catalysts prepared by nanoengineered synthesis and processing routes. One example involves thermal treatment to manipulate the surface and strain properties of PtmM1nM2100-m-n (M1, M2 = Ni, Co, V, Fe, W, etc) nanoparticle electrocatalysts. In addition detailed characterization of the lattice structures using XRD and XAS techniques, the electrocatalytic activities were compared with those obtained with commercially-available platinum catalysts. The results revealed new insights into the understanding of how the strain and surface properties of the trimetallic nanoparticle catalysts influence the electrocatalytic activity and stability. Implications of the findings to the design of highly active and stable trimetallic catalysts for fuel cell applications will also be discussed.
3:30 PM - GG2.5
Modification of Gold Catalysis with Aluminum Phosphate as an Overlayer for Oxygen-reduction Reaction.
Yejun Park 1 , Seunghoon Nam 1 , Yuhong Oh 1 , Jungjin Park 1 , Byungwoo Park 1
1 Material Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractThe catalytic nature of gold has been found to be tunable through the control of nanoparticle size, the combination of matrix materials, and the nanostructures of composites. As a previous research, catalytic activities of the Au/AlPO
4 nanocomposites were examined for oxygen-reduction reaction (ORR). It is important to distinguish the alteration in the size or surface facets of gold from the combinational modification with aluminum phosphate. In this research, aluminum phosphates are deposited on gold catalysts with simple electrode geometry, and the activity of gold catalyst with/without aluminum phosphate is examined. The ratio of gold surface facets can affect the ORR activity. Using the underpotential deposition of lead on gold, it is observed that the surface facets of gold are unchanged during the formation of aluminum-phosphate overlayer on gold catalysts. More feasible reactions existed within the region of approximately 0.8 – 1.0 V (vs. reversible hydrogen electrode) in the alkaline media. These feasible reactions are associated with hydrogen peroxide intermediates, and the involved mechanisms will be discussed in this talk. [1] Y. Park, B. Lee, C. Kim, J. Kim, S. Nam, Y. Oh, and B. Park
J. Phys. Chem. C 114, 3688 (2010). [2] Y. Park, B. Lee, C. Kim, J. Kim, and B. Park,
J. Mater. Res. 24, 140 (2009). Corresponding Author: Byungwoo Park:
[email protected]3:45 PM - GG2.6
Fe-Porphyrin Embedded Carbon Nanotubes as Efficient Oxygen Reduction Catalysts: Theory and Experiment.
Yong-Hyun Kim 1 , Duck Hyun Lee 2 , Won Jun Lee 2 , Won Jong Lee 2 , Sang Ouk Kim 2
1 Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 2 Department of Materials Science and Engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractNatural complexes of transition metal-N4 macrocycles, such as Fe-porphyrins and Fe-phthalocyanines, have been considered as oxygen reduction catalysts for low-temperature fuel cells. So far, the promising Fe-porphyrin-based Fe-N4 catalysts have not yet demonstrated an acceptable performance in the oxygen reduction reaction (ORR). This is primarily because the simple blending or chemical functionalization of Fe-porphyrins with carbon supports (carbon black or carbon nanotubes) results in a low density of active catalytic sites and poor electrical, thermal, mechanical contacts with the carbon supports. Very recently it was suggested, based on results of first-principles density-functional theory (DFT) calculations, that the Fe-N4 unit could be effectively incorporated in a large scale into graphenes by defect engineering, but without losing its original chemical activities [1,2]. The seamless connection of Fe-N4 with the sp2 graphitic network may be suitable for overcoming the current ORR limitations of the Fe-porphyrin-based technology.Here we report results of total energy and electronic structure analyses of Fe-N4 embedded carbon nanotubes and their formation mechanisms based on DFT calculations. We reveal that the special nanotubes can provide efficient pathways for ORR, comparable to those of Pt catalysts. We will also compare the theoretical findings with experiment results of Fe-porphyrinic nanotubes, including x-ray and ultraviolet photoemission spectra and cyclovoltammetry and rotating disk electrode analyses for their ORR performance [3]. [1] Y.-H. Kim, Y. Y. Sun, W. I. Choi, J. Kang, and S. B. Zhang, Phys. Chem. Chem. Phys. 11, 11400 (2009).[2] W. I. Choi, S.-H. Jhi, K. Kim, and Y.-H. Kim, Phys. Rev. B 81, 085441 (2010).[3] D. H. Lee, W. J. Lee, W. J. Lee, Y.-H. Kim, and S. O. Kim, submitted (2010).
4:30 PM - GG2.7
Lower Cost Gas Diffusion Media Based on Alternative Carbon Fiber Precursors: Properties and Proton Exchange Membrane Fuel Cell Performance.
Paul Nicotera 1 , Robert Evans 2 , Chunxin Ji 1 , Christopher Weaver 2 , Po-Ya Chuang 1
1 Electrochemical Energy Research Lab, General Motors, Honeoye Falls, New York, United States, 2 , Engineered Fibers Technology, LLC, Shelton, Connecticut, United States
Show AbstractConventional carbon fiber Diffusion Media (DM) is relatively expensive, primarily due to the high strength, small tow polyacrylonitrile (PAN) based carbon fibers and high heat treatment temperatures used in its production. The present study shows that DM made with a less expensive carbon fiber type can maintain fuel cell performance while lowering DM cost by an estimated 10-15%. Previous work has demonstrated the feasibility of stably carbonizing textile-grade PAN fibers through chemical and radiation pre-treatments [1]. The properties and fuel cell performance of DM comprised of alternative, experimental carbon fibers, as well as commercially available lower cost carbon fibers, are presented. The lab-scale DM samples indicate potential for further cost savings by reducing the number of papermaking steps. Combined with less expensive heat treatments [2], significantly cheaper DM could contribute to reducing the overall PEM fuel cell stack cost and enabling automotive application.[1] C.D. Warren et al, “Development of Commodity Grade, Lower Cost Carbon Fiber–Commercial Applications,” SAMPE Journal, March/April 2009.[2] R. Evans et al, “The Role of Heat Treatment Temperature in the Properties and Performance of PEM Fuel Cell Diffusion Media,” Materials Science & Technology 2009 Conference (Oral Presentation), October 2009.
4:45 PM - GG2.8
Polyaniline and Polypyrrole as Cation Repelling Layers: A Study on Nafion® Membrane Permeability.
Birgit Schwenzer 1 , Soowhan Kim 1 , Vijayakumar Murugesan 1 , Z. Gary Yang 1 , Jun Liu 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractDespite its low cation selectivity, Nafion (N117) is still the most commonly used cation exchange membrane in fuel cells and other applications that require membranes with high proton conductivity and good chemical stability, for example redox flow batteries. This study discusses the approach of increasing the permselectivity of Nafion by coating the membrane with polymers containing heteroaromatic groups that under acidic conditions act as a cationic shielding layer. Our findings contribute to the basic understanding of Nafion’s interaction with other polymers and therefore are of interest to scientist working with Nafion for a wide variety of applications. Various analysis techniques have been used to elucidate molecular binding and morphological differences between as-purchased N117 and two model systems, polyaniline/N117 and polypyrrole/N117 composite membranes. Both materials combinations of the composite membranes have been investigated in part before, but we, for the first time, correlate the influence of reaction parameters of the polymerization reaction on VO2+ diffusivity and area specific resistance of the composite membranes. While for polypyrrole/N117 membranes no visible coating layer is detectable by cross-section SEM whereas polyaniline/N117 samples show thick coating on the N117 surfaces, the comparatively higher area specific resistance and lower diffusivity observed for polyaniline/N117 membranes cannot exclusively be attributed to physical pore size restriction or blocking by the polyaniline film. IR and NMR spectroscopy reveal different binding strengths between the heteroaromatic group and the sulfonyl groups of N117 for the two polymer coatings. The observed increase in area specific resistance can be correlated to increased unavailability of sulfonyl groups for proton conduction.
GG3: Materials for Fuel Cells I
Session Chairs
Monday PM, November 29, 2010
Back Bay A (Sheraton)
5:00 PM - GG3.1
HPW/Silica Inorganic Membrane Based on Mesoporous Silica for Fuel Cells.
Jie Zeng 1 , Sanping Jiang 1
1 MAE, Nanyang Technology Univ, Singapore Singapore
Show AbstractAlthough proton-exchange-membrane fuel cells including PEMFC and DMFC have been known for a long time, they have not reached large-scale development as some issues are still unresolved. These are mainly related to limited functional characteristic of the proton exchange membrane. The state-of-the-art proton exchange membrane is the sulphonated tetrafluoroethylene copolymer Nafion developed by DuPont in the late 1960s, whose conductivity relies on the humidity level and suffers serious proton conductivity loss at temperatures above 120oC due to the membrane dehydration under high temperature and low relative humidity environment. These issues many be addressed to an extent to switching to inorganic materials based membranes because of their inherent mechanical and thermal stability.In this paper, a novel proton exchange membrane using phosphotungstic acid as proton carrier and mesoporous silica as framework material was successfully developed for the application of high temperature H2/air and methanol/air fuel cells. The results show that host material of the HPW/meso-silica with pore diameter of 5.0nm has the highest proton conductivity of 0.11Scm-1 at 90oC/100%RH with an activation energy of ~10.0 kJmol-1. The proton exchange through a hydrogen-bonding interaction via HPW molecules is also confirmed by a computational modeling using density functional theory. A PEMFC based on a HPW/meso-silica membrane produced a power output of 174mWcm-2 under 130oC/20%RH in H2/air and of 122mWcm-2 under 160oC/8%RH in methanol/air. The results indicate that the HPW/meso-silica is a promising electrolyte for proton exchange membrane fuel cells operating at elevated high temperatures. The present study demonstrates that inorganic proton exchange membranes with high proton conductivities can be realized in the HPW/meso-silica composite, and demonstrates for the first time that the three-dimensional mesoporous silica based composite can be used as effective proton exchange membrane for high temperature H2/air Fuel Cells and DMFCs.
5:15 PM - GG3.2
Proton Conduction in Acid Containing Organic Ionic Plastic Crystal based Electrolytes and Membranes.
Usman Ali Rana 1 4 , Maria Forsyth 1 4 , Douglas MacFarlane 2 4 , Masayoshi Watanabe 3
1 Materials Engineering, Monash University, Melbourne, Victoria, Australia, 4 ARC Center of Excellence for Electromaterial Science (ACES), Monash University, Melbourne, Victoria, Australia, 2 School of Chemistry, Monash University, Melbourne, Victoria, Australia, 3 Department of Chemistry and Biotechnology, Yokohama National University, Yokohama Japan
Show AbstractProton conduction in mixtures of organic ionic plastic crystals and various acids has been investigated with the aim to develop a new class of proton conducting electrolytes for low or medium temperature PEM fuel cells. High thermal stability coupled with high proton conductivity has been achieved by the addition of H3PO4 and HTFSI in Choline dihydrogen phosphate ([Choline][DHP]). Isothermal TGA indicates high thermal stability and no significant weight loss up to 200 °C in these systems. Investigation of phase behaviour in these materials using Differential Scanning Calorimetry (DSC) showed various solid-solid phase transitions in pure as well acid containing samples. The electrochemical impedance spectroscopy (EIS) showed conductivity in the range of 10-3 to 10-2 S cm-1 in the temperature range of 110 to 120 °C. The proton activity in these materials is verified by electrochemical characterization technique, where the large cathodic currents due to proton reduction clearly indicate proton conductivity in these materials. Pulsed field gradient NMR was undertaken in some cases and suggests that the proton diffusion is significantly higher than the other species present in the acid [Choline][DHP] mixtures. Freestanding membranes were fabricated by impregnating the acid [Choline][DHP] mixtures in cellulose acetate membranes. The membranes showed high ionic conductivity and high thermal stability especially in the temperature range of interest (~110 °C). Cyclic voltammetery experiments in the case of membrane materials also showed significant H+ reduction currents. Finally the membranes were tested in a fuel cell configuration.
5:30 PM - GG3.3
Proton-conducting Metal-organic Frameworks.
Jamie Ford 1 2 , Jason Simmons 2 , Hui Wu 1 3 , Wei Zhou 1 3 , Taner Yildirim 2 1
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 , NIST Center for Neutron Research, Gaithersburg, Maryland, United States, 3 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States
Show AbstractVehicles powered by polymer electrolyte membrane (PEM) fuel cells are an exciting alternative to current fossil fuel technology. The membranes in these cells serve as both charge transporter, ferrying protons from the anode to the cathode, and gas diffusion barrier, preventing the backflow of oxygen to the anode. Currently, hydrated sulfonated polymers are the preferred material for these membranes. The presence of water, however, limits the operating temperature to 100 C, reducing the electrode kinetics and CO tolerance of the entire system. In an effort to increase the efficiency and operating temperature of these fuel cells, we are investigating the proton conductivity of new host/guest materials based on metal-organic frameworks (MOFs) loaded with proton transporters, such as imidazole or solid acid salts. These thermally stable frameworks provide well-defined pores that accommodate the formation of guest networks to generate proton-conducting pathways. Here, we will present the structure and proton dynamics of these materials as elucidated by impedance spectroscopy and neutron scattering measurements. We will also discuss how pore size, pore geometry, and guest and pore chemistries affect the overall conductivity of the systems.
5:45 PM - GG3.4
Fabrication and Electrical Properties of Ceria-nanoparticles Monolayer.
Hitoshi Takamura 1 , Kazuki Tamura 1
1 Department of Materials Science, Tohoku Univ., Sendai Japan
Show AbstractMicro-fabricated solid oxide fuel cells (μ-SOFCs) have been attracting much attention because of reducing operating temperature and widening application area. To prepare a thin solid-electrolyte film such as Y-stabilized zirconia and acceptor-doped ceria on a silicon wafer, to date, sputtering, pulsed-laser deposition (PLD), atomic-layer deposition (ALD) techniques have been utilized. In this study, as a novel technique to prepare ultra-thin and uniform oxide films, Langmuir-Blodgett (LB) deposition of ceria nanoparticles is examined. The purpose of this study is to prepare LB film comprising of ceria-nanoparticles and to investigate its electrical properties by using a scanning-probe microscope (SPM). Colloidal ceria nanoparticles were prepared by emitting KrF-excimer laser to cerium nitrate solution with oleic acid as a dispersant agent. The resultant ceria-nanoparticles (D = 3 - 4 nm) colloids were dispersed on a subphase of water at 278 K. The status of ceria nanoparticles on the subphase was observed by using a Brewster-angle microscope. The LB monolayer film comprising of ceria nanoparticles was then deposited onto a silicon wafer at a surface pressure of 15 to 20 mN/m. By using SPM, a thickness of the uniform LB film was found to be approximately 4 nm, showing a good agreement with a diameter of ceria nanoparticles. The formation of dense ceria-nanoparticles monolayer was also confirmed by TEM observation. Prior to electrical conductivity measurements, ozone ashing was performed to remove surfactants. Electrical properties such as I-V characteristics and ac-impedance of the ceria-nanoparticles monolayer were evaluated by SPM. In the presentation, in addition to ceria nanoparticles, preparation and electrical properties of LB films of Sm-doped ceria nanoparticles will be discussed.
Symposium Organizers
Ting He ConocoPhillips
Karen Swider-Lyons Naval Research Laboratory
Byungwoo Park Seoul National University
Paul A. Kohl Georgia Institute of Technology
Harry L. Tuller Massachusetts Institute of Technology
GG4: Materials for Fuel Cells II
Session Chairs
Tuesday AM, November 30, 2010
Back Bay A (Sheraton)
9:30 AM - GG4.1
Boundary Effects on the Electrical Conductivity of Cerium Oxide Thin Films.
Marcus Goebel 1 , Giuliano Gregori 1 , Joachim Maier 1
1 Maier, Max-Planck-Institute for Solid State Research, Stuttgart Germany
Show AbstractIn the present contribution, we report on a systematic study about the impact of grain boundaries on the overall electrical conduction properties of ceria thin films. For this purpose, ceria films with different microstructures were fabricated using pulsed laser deposition (PLD) on a variety of substrates. This includes also thin films with ultra small grains prepared with PLD at low temperatures. In order to additionally investigate on how grain boundaries affect both the ionic and the electronic conductivity, samples with different doping contents were prepared. The electrical conductivity of the thin films was measured as a function of temperature and oxygen partial pressure (pO2) in a specially constructed measurement cell under identical conditions using electrochemical impedance spectroscopy.Notably, in all measured samples the grain boundaries were found to influence the electrical conductivity considerably. In particular, for the films with an ultra small grain size, changes in the conductivity up to several orders of magnitude were observed. For all polycrystalline films investigated, a decrease of the ionic conductivity was found while the electronic conductivity was increased compared with epitaxial samples. Also for the activation energies significant changes with microstructure were observed. Here with increasing grain boundary density the activation energies of the ionic charge transport were systematically increased while the activation energies of the electronic conductivity were decreased. Remarkably, this shift from ionic to electronic conduction is so pronounced that, even for the heavily acceptor doped films (10 mol% Gd-doped) with ultra small grain size, a pO2 dependence could be observed at low temperatures, that indicates a partial electronic conductivity. The results are discussed in the framework of the general space charge model.
9:45 AM - GG4.2
Electrical Conductivity and Phase Stability of Ca-doped LaNb1-xTaxO4.
Zhonghe Bi 1 , Jung Hyun Kim 2 , Craig Bridges 1 , Ashfia Huq 2 , M. Parans Paranthaman 1 , Matthew Stone 2
1 Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractHigh-temperature proton conducting materials have a wide range of technological application in fuel cells, gas separation, gas sensors, and hydrogenation/dehydrogenation of hydrocarbons, etc. In recent years, the Ca-doped Rare-Earth (RE) ortho-niobates have been reported by Haugsrud and Norby, which represent the highest levels of proton conductivity of 0.001 S/cm, in oxides without Ba or Sr as primary components. For Ca-doped RENbO4, however, one disadvantage is that their thermal expansions are strongly influenced by the phase transition from low-temperature monoclinic to high-temperature tetragonal polymorph. According to previous studies, the phase transition temperatures increase with decreasing radii of the Rare-Earth cations in the temperature range of 500-830°C, which are much lower than for the isostructural RETaO4 (1300-1450°C). Partial substitution of Nb with Ta offers a way to change the phase transition temperature in order to increase the temperature range at which these materials will prove to be stable. To this end we have undertaken a systematic study of this family of materials of Ca-doped LaNb1-xTaxO4 (x=0 to 1 in steps of 0.1). We use (a) neutron diffraction to study its crystallography, structural phase transition, presence of anionic vacancy; (b) inelastic neutron scattering to understand softening of phonons; and finally (c) AC impedance technique to characterize its electrochemical properties in order to evaluate its suitability as an electrolyte material for high-temperature electrochemical device application. Research sponsored by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (MSED), ORISE, ORAU, and ORNL’s LDRD program, under contract DE-AC05-00OR22725 with UT-Battelle, LLC managing contractor for Oak Ridge National Laboratory.
10:00 AM - GG4.3
Controlled Synthesis of Nanocomposite Materials Using a High Pressure, High Temperature Continuous Microfluidic Reactor Platform.
Victor Sebastian Cabeza 1 , Soubir Basak 1 , Klavs Jensen 1
1 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMaterials in nanometer size range show improved and novel chemical, electrical, mechanical and optical properties, compared to bulk materials. Most often, these properties are function of particle size, shape, morphology and chemical composition of the nanomaterial. There is a continuous effort to enhance and optimize these property of nanomaterial targeted for its applications like catalysis, sensors, contrast agent for medical imaging, photovoltics and fuel cells. Synthesis of these nanomaterials is a mostly complex multistep batch process through colloid chemistry. It is typically difficult to reproduce size, shape and chemical composition of nanoparticles from batch to batch. Further, it is desirable to vary a parameter of the nanomaterial keeping other parameters constant to study the effect of that parameter on the overall performance. Thus, it is often necessary to develop a new process to synthesize nanomaterials with particular characteristics by using conventional batch synthesis. Continuous flow synthesis would address problems of reproducibility inherent in batch reactors and allow scale up the amount of nanomaterial synthesized. Continuous flow-based synthesis methods operate at steady state and offer superior control over reaction conditions, such as reagent addition, mixing, and temperature. A continuous microfludic device (microreactor) setup has been designed to operate at high pressure and high temperature as a platform for synthesis of nanomaterials. By tuning the process parameters such as precursor concentration, reaction temperature, reaction residence time, and solvent, it is possible to rapidly synthesize nanocomposite materials in a single step with tunable properties and systematically study the effect of size, shape, morphology and chemical composition on the performance of the nanomaterial on fuel cell. Synthesis of elemental and bimetallic nanomaterials of various size, shape, morphology and chemical composition is demonstrated using the microreactor. Elemental Pt nanoparticles of ~3nm size were synthesized by reducing K2PtCl4 with ethylene glycol, using PVP as surfactant at 150oC at 60 s residence time. Dendrimer shape nanostructures made of single Pt or Pt-Pd composites were synthesized by controlled reduction using ascorbic acid. Application of high pressure inside the microreactor is particularly effective, as it allows using a solvent beyond the limit of its boiling point. Based on the benefit of high pressure microreactor system, water is chosen as a solvent for most of the synthesis process even for a reaction conditions above 100oC by applying a small back pressure (~2MPa), turning the process into a green process. Synthesized bimetallic nanodendrimer particles show improved catalytic performance over commercial catalyst in fuel cell. Systematic studies of the catalytic property of bimetallic nanocomposite become feasible using a common benchmark platforms, specifically, electrochemical analysis.
10:15 AM - GG4.4
Electrical Measurement of Praseodymium-Cerium Oxide Films.
Di Chen 1 , Sean Bishop 1 , Pyeong-Seok Cho 1 , Jae Jin Kim 1 , Harry Tuller 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractCeria is used in the three-way automotive exhaust catalysts given its ability to store and release oxygen in oxidizing (e.g. NOx ) and reducing atmospheres (e.g. CxHy, CO), respectively. The redox reaction rate depends on the oxygen ion conduction through the bulk, which in turn depends on oxygen vacancy mobility and concentration, as well as the surface exchange rate of oxygen, which has been correlated to the concentration of both electronic and oxygen vacancy defect concentrations. Praseodymium cerium oxide (PrxCe1-xO2−δ) is a potential TWC oxygen storage material given its mixed ionic-electronic conducting (MIEC) character together with high levels of nonstoichiometry under relatively oxidizing (e.g. air) conditions. In this study, well defined and reproducible, dense praseodymium-cerium oxide thin films with different Pr levels were prepared by pulsed layer deposition on single crystal substrates, and their electrochemical and catalytic properties examined by impedance spectroscopy. The results are discussed in relation to PrxCe1-xO2−δ ‘s defect chemistry.
10:30 AM - **GG4.5
Materials for Fuel Cells: Strategies, Adjusting Screws and Limitations.
Joachim Maier 1
1 Solid State Chemistry I, MPI for Solid State Research, Stuttgart Germany
Show AbstractFirst, the contribution discusses materials requirements for high temperature, intermediate temperature and low temperature fuel cells. Second, the possibility to generate new materials are discussed. Beyond the possibility of introducing new structures and compounds (a few examples are given), it is particularly the possibility to modify established materials that is to the fore. Modification of given materials can be done by homogeneous and heterogeneous doping, i. e. by increasing chemical and morphological complexity. Recently, the possibility of size-variation (nanostructuring) came into the focus of interest. Possibilities and limitations of these adjusting screws with respect to fuel cell applications and relevant examples are discussed.
11:30 AM - **GG4.6
Novel Smart Catalysts for Next Generation of PEM Fuel Cells and Li-Air Batteries.
Sanjeev Mukerjee 1
1 Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts, United States
Show AbstractOxygen reduction reaction (ORR) is a critical component for most fuel cells and in some industrial electrolysis processes such as chlorine generation. In addition it is also important from the perspective of some emerging technologies such as the Li-Air batteries. In all of these the interaction of the active reaction center with molecular oxygen in the context of competing surface processes play an important role in determining the overpotential and kinetics of the reaction. Our group has played an important role in understanding this complex phenomenon in a wide variety of electrolytes, interfaces and applications. This presentation will provide a perspective in terms of our new results where our focus has been on comparing ORR in conventional Pt and Pt alloy nano-particles in aqueous acid systems to more complex environments such as chalcogenides, metal polymer composites and enzymatic reaction centers. In this, results from a mix of electrochemical measurements and in situ synchrotron x-ray absorption methods will be described. Among the materials to be presented will be our latest results on metal organic composite electrocatalysts and laccase enzymatic reaction centers in the context of aqueous acid, alkaline and bio-fuel cells as well as electrocatalysis in non aqueous environments in the context of metal organic systems for Li-Air battery applications.Newly developed method referred to as the ‘Delta mu Technique’ based on near edge component (X-ray absorption near edge spectra, XANES) of synchrotron in situ XAS provide unprecedented ability to map the adsorption of various species on an electrochemical interface as a function of operating cell conditions. This method has now been extensively validated on a variety of electrode interfaces1-5. This presentation will provide important links between electrocatalytic pathways and reaction centers for future materials design projections.Acknowledgements:The in situ XAS was run at Beamlines X11a, X19 and X18 at NSLS located at Brookhaven National Laboratory (BNL) in Upton, NY which is supported by the Department of Energy (DOE). Authors gratefully acknowledge the financial support from the Army research office via both a single investigator and a multi-university research grant.References:(1)Teliska, M.; Murthi, V. S.; Mukerjee, S.; Ramaker, D. E. J. Electrochem. Soc. 2005, 152, A1259-A2169.(2)Teliska, M.; O'Grady, W. E.; Ramaker, D. E. Journal of Physical Chemistry B 2004, 108, 2333-2344.(3)Teliska, M.; O'Grady, W. E.; Ramaker, D. E. Journal of Physical Chemistry B 2005, 109, 8076-8084.(4)Teliska, M.; Ramaker, D. E.; Srinivasamurthi, V.; Mukerjee, S. In Fundamental Understanding of Electrode Processes in Memory of Professor Ernest B. Yeager; Prakash, J., Ed.; The Electrochemical Society: Orlando, FL, 2003; Vol. PV 2003-30, p 212-216.(5)Ziegelbauer, J. M.; Gatewood, D.; Gullá, A. F.; Ramaker, D. E.; Mukerjee, S. Electrochemical and Solid State Letters 2006, 9, A 430
12:00 PM - GG4.7
Catalytic Activity of Metal Coated NanoFilms for Hydrogen Generation.
Melik Demirel 1 2
1 Engineering Science, Penn State University, University Park, Pennsylvania, United States, 2 Wyss Institute, Harvard University, Boston, Massachusetts, United States
Show AbstractHydrogen is envisioned as the next-generation fuel to provide a clean alternative to the current hydrocarbon-based fuels in the field of transportation and even personal electronics. Metals, such as platinum, ruthenium, cobalt, and nickel, act as a catalyst to produce hydrogen gas from metal-hydride solutions. Particularly, Cobalt is an inexpensive material compared to platinum, and has a relatively high catalytic activity compared to nickel for hydrogen generation. Here, we report the catalytic activity of a cobalt coated polymeric nanofilms in NaBH4 solutions. The hydrogen release rate of the cobalt-coated polymeric nanofilm shows a rate of 4250 mL/(g.min) (i.e., rate of hydrogen gas per cobalt mass at room temperature and pressure), which is comparable to the values obtained on platinum, and ruthenium systems. The nanofilms can be reused without any significant reduction in catalytic activity.
12:15 PM - GG4.8
Halogenated, Metallated Polypyrroles as Catalysts for Catalytic Oxidation of CO Poisons in Fuel Cell Feedstreams.
Jeremy Pietron 1 , Justin Biffinger 2 , Syed Qadri 3
1 Surface Chemistry Branch, Code 6170, Naval Research Laboratory, Washington, District of Columbia, United States, 2 Chemical Dynamics and Diagnostics Branch, Code 6110, Naval Research Laboratory, Washington, District of Columbia, United States, 3 Material Science and Technology, Code 6300, Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractMetallated polypyrroles are moderately active electrocatalysts in proton exchange membrane fuel cells (e.g.: R. Bashyam, P. Zelanay Nature 443 (2006) 63–66; A. L. M. Reddy, N. Rajalakshmi, S. Ramamprabhu Carbon 46 (2008) 2–11; M. Yuasa, A. Yamaguchi, H. Itsuki, K. Tanaka, M. Yamamoto, K. Oyaizu Chem. Mater. 17 (2005) 4278–4281). Their catalytic activity derives from pyrrole-coordinated metal centers similar to those in metallated porphyrins. Modification of porphyrins with electron-withdrawing halogen atoms renders their metallated form highly selective for the oxidation of carbon monoxide (CO) in the presence of hydrogen. Metallated, halogenated polypyrroles should be similarly selective catalysts and are inexpensive, easily processed conductive polymeric materials, making them suitable for electrocatalytic removal of CO from reformate feedstreams with simultaneous power generation. We have developed a general method for (a) depositing halogenated polypyrroles on conductive carbon supports, and (b) inserting Co(III), Rh(III), and Fe(III) into the polymer. We have synthesized perfluorinated polypyrrole (F-ppyr), deposited it on high-surface-area carbon supports, metallated the deposited polymer with cobalt(II) acetate, and characterized the composite (Co-F-ppyr/C) with TGA, DSC, XRD and XPS. While Co-F-ppyr/C is active for oxygen reduction, the current methods for deposition have not rendered an active carbon monoxide oxidation catalyst. Initial results suggest that CO binds very strongly to the electron-poor metal center in Co-F-ppyr/C. In addition, we have synthesized 3,4-dichloropyrrole as a precursor to perchlorinated polypyrrole (Cl-ppyr). Ab initio results indicate that Cl-ppyr will be the most selective catalyst for CO binding compared to non-halogenated, fluorinated and brominated polypyrroles. The flexibility in choice of both metal centers and of the electron-withdrawing power of the polypyrrole comprises a powerful approach to a new class of selective electrochemical redox catalysts with highly tunable properties.
12:30 PM - GG4.9
Enhanced CO Oxidation Catalysis of Pt0.1Cu0.9/Fe2O3 Synthesized by Radiolytic Process.
Takao Yamamoto 1 , Ryota Kitagawa 1 , Satoshi Seino 1 , Takashi Nakagawa 1
1 Graduate School of Engineering, Osaka University, Suita, Osaka, Japan
Show AbstractHydrogen gas fed to the polymer electrolyte fuel cell (PEFC) should be free from CO which poisons Pt in its anode, so that reformed hydrogen is purified usually by a catalyst which preferentially oxidizes CO. Requirements for the catalyst are lower cost and higher activity especially in a temperature range suitable for PEFC operation. In this paper we report on a new catalyst containing Pt only of 10 at% and balancing Cu supported on Fe2O3 , which exhibits a higher CO oxidation performance in 80-120°C than the commercial catalyst. The catalyst samples were synthesized by the radiolytic method we have developed [1, 2]. This method is a simple one-pot process, in which a glass vial containing water together with support powder (γ-Fe2O3 ) and metal sources (H2PtCl6 and CuSO4) is irradiated with an electron beam (4.8 MeV, 20 kGy) only for several seconds. Radiation induced radicals reduces the aqueous ions of Pt and Cu and induces precipitation of composite phases on the support. By varying loadings of Pt and Cu, several samples were obtained with different compositions. These samples were characterized by techniques of XRD, XAFS, TEM with EDS and ICP. The composite catalyst seems to consist of Pt-Cu bimetallic and Cu oxide. The sample powder was charged in a glass tube heated at a temperature in a region of 60 - 200°C, through which a gas mixture (1%CO, 0.5%O2, 67.2%H2 and N2) flowed. CO and CO2 concentrations in the outlet gas were measured by a gaschromatgraph to evaluate CO oxidation catalytic activity. For comparison, activity of commercial catalyst, TSSB-5 (Tanaka Kikinzoku Kogyo K.K.), was measured as well. As the Pt-loading decreased down to 50 at%, the activity decreased and with increasing temperature it decreased. However, as the Pt-loading decreased lesser than it, the activity contrariwise increased and with increasing temperature up to 100°C it increased. In the temperature region of 60-100°C, the present sample containing only 10at% Pt showed a significantly higher activity than the commercial TSSB-5. This new material is promising for use as a catalyst purifying hydrogen gas which is fed to PEFC.[1] S. Seino et. al.; Journal of Nanoparticle Research,10 (2008)1071-1076.[2] T. A. Yamamoto et. al.; Mater. Res. Soc. Symp. Proc. Vol. 1217 (2001) 1217-Y08-03.
12:45 PM - GG4.10
AuPt Nanoparticles with Tuable Pt Surface Coverage.
Zhichuan Xu 1 2 , Jin Suntivich 1 , Junhyung Kim 1 , Chris Carlton 1 , Hubert Gasteiger 1 , Yang Shao-Horn 1 , Kimberly Hamad-Schifferli 1 2
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractBimetallic nanoparticles with controlled size, composition, and structure have gained considerable interest for fuel cell applications. Their catalytic activity is strongly dependent on their surface chemistry. Much effort has been devoted to exploring synthetic methodologies for making a Pt layer over nanoparticles, which are appealing for fuel cell related reactions. However, coating Pt onto metallic particles to create core/shell nanoparticles has been difficult to achieve. Here, we demonstrate a simple wet-chemical synthesis of AuPt bimetallic nanoparticles with a tunable Pt surface coverage. As-synthesized AuPt nanoparticles were characterized by TEM, XRD, and UV-Vis. The Pt surface was found by cyclic voltammetry and EDX mapping. Their catalytic activities toward oxygen reduction and methanol oxidation were compared with pure Au and Pt nanoparticles. The method offers a simple strategy for making core/shell bimetallic nanoparticles with controlled surface chemistry. It enables a good connection between catalyst design and modern catalysis concepts.
GG5: Materials for Fuel Cells III
Session Chairs
Tuesday PM, November 30, 2010
Back Bay A (Sheraton)
2:30 PM - GG5.1
Delta-Bi2O3 Stabilized by Epitaxial Growth on Single Crystal Oxide Substrates.
Danielle Proffit 1 2 , Guo-Ren Bai 1 , Dillon Fong 1 , Timothy Fister 1 , Stephan Hruszkewycz 1 , Matthew Highland 1 3 , Peter Baldo 1 , Paul Fuoss 1 , Thomas Mason 2 , Jeffrey Eastman 1
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 3 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractOxide ion conductors are critical components in many important energy conversion devices, including solid oxide fuel cells (SOFCs), oxygen separation membranes, and oxidation catalysts. Cubic fluorite delta-Bi2O3 is known to have the largest ionic conductivity of any oxide material; however, it is stable only between 725 and 825°C, limiting its practical application [1]. Previous work has stabilized the delta phase to room temperature using dopants; however, dopants are known to induce oxygen ordering and degrade the conductivity. Fabrication of delta-Bi2O3 into heterostructures has proven that the delta phase can be protected from reduction at low oxygen partial pressures [2]. Using an alternate approach, we ehave stabilized the high temperature phase to room temperature using epitaxial growth on single crystal oxide substrates. We have grown nanostructures of delta-Bi2O3 on (001) oriented SrTiO3 and (110) oriented DyScO3 single crystal substrates, controlling the aspect ratio by varying the miscut of the substrate [3]. We found that the delta phase can also be stabilized as continuous thin films by epitaxial growth on (111) Y2O3-stabilized ZrO2 or (0001) alpha-Al2O3. Characterizing these structures with the in-situ capabilities of synchrotron x-ray scattering provides a unique method of probing the strain and phase stability of these thin films and nanostructures under conditions relevant to device operation. Our studies have observed the structure at controlled temperatures and oxygen partial pressures, confirming that the delta-Bi2O3 nanostructures are coherently strained to the substrates at room temperature. Results show that the nanostructures exhibit a superstructure, possibly associated with oxygen ordering. Annealing the nanostructures at 600°C causes gradual conversion of the (001) oriented delta phase to an unidentified strain-relaxed phase. Ongoing work uses in situ electrical measurements at the synchrotron to help determine the origin of the observed superstructure and its effect on conduction behavior. Ramifications for achieving superionic conductivity in thin films and heterostructures will be discussed.[1] N.M. Sammes et al., J. Europ. Ceram. Soc. 19 (1999) 1801.[2] Jun-Young Park et al., J. Am. Ceram. Soc. 88 (2005) 2402. [3] D. L. Proffit et al., Appl. Phys. Lett. 96 (2010) 021905.
2:45 PM - GG5.2
PT Thin Films ON YTTRIA Stabilized Zirconia: Morphology and Electrochemical Performance.
Thomas Ryll 1 , Henning Galinski 1 , Lukas Schlagenhauf 1 , Felix Rechberger 1 , Anja Bieberle-Huetter 1 , Jennifer L.M. Rupp 1 , Ludwig Gauckler 1
1 Department of Materials, ETH Zurich, Zurich Switzerland
Show AbstractMetal thin films deposited on ceramic surfaces are usually thermodynamically unstable. By means of thermally activated diffusion hole formation, hole growth and finally the coarsening of isolated islands take place. The entity of these processes is known as agglomeration or dewetting. An important technological application of agglomerated metallic thin films are solid oxide fuel cell electrodes, in terms of which a long three phase boundary (tpb), where air, electrode and electrolyte are in contact, is critical. This study builds a bridge between the morphology of agglomerated Pt thin films and their performance with respect to the oxygen reduction reaction on yttria stabilized zirconia (YSZ) substrates.Platinum thin films with thicknesses from 15 to 360 nm were sputtered on YSZ substrates and subsequently annealed for 2 h between 650 and 800°C in air. The morphological evolution during annealing was investigated by scanning electron microscopy. Electrochemical impedance spectroscopy provided information about the electrochemical performance of the agglomerated Pt thin films. Depending on film thickness and annealing temperature, film morphologies belonging to hole formation, hole growth as well as particle growth were distinguished. While the exposed substrate area exhibited a sigmoidal dependency on the annealing temperature, the tpb-lengths in the order of 100 m/cm2 passed through a maximum. Both dependencies shifted to higher temperatures with increasing film thickness. The electrochemical characterization of agglomerated Pt thin films provided evidence for a linear correlation between electrode conductance and tpb-length. At the same time Pt thin film electrodes that were not subjected to heat treatments and consequently did not feature any visible tpb, partly showed higher electrode conductances than agglomerated thin films. These, at first sight, conflicting results were explained by a model, assuming that the electrode conductance of Pt thin film electrodes is additively composed of a basic conductance resulting from defects like pinholes, voids or grain boundaries and a conductance due to the visible and quantifiable tpb of agglomerated or micro-patterned thin film electrodes. It will be shown that this model can be used to reconcile literature data. Furthermore, an outlook will address the electrochemical performance of de-alloyed Pt electrode thin films featuring an open porosity in the nanometer range and, therefore, orders of magnitude higher basic conductances compared to agglomerated thin films.
3:00 PM - GG5.3
Surface Modified Thin Film Perovskite Microelectrodes on YSZ – An Electrochemical Impedance Study.
Eva Mutoro 1 , Ethan Crumlin 1 , Yang Shao-Horn 1
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe efficiency of state-of the art SOFCs (solid oxide fuel cells) is limited by the ORR (oxygen reduction reaction) at the cathode. Considering a specific electrode material, a compositional modification of its electrode surface can influence the electrode kinetics – either by altering the number of active reaction sites, or changing their (catalytic) activity. Potentially, thin surface coverages can enhance the electrode activity, and thus contribute to higher efficiency of SOFCs.In addition a number of studies reported compositional changes of AxSr1-xBO3-type perovskite electrodes during operation at elevated temperature, causing a time-dependant change of the electrode performance. Often Sr-segregation has been observed, but also A and B enriched surfaces – depending on the experimental conditions. The multitude of influencing parameters for a composition change (e.g. temperature, oxygen partial pressure, morphology/geometry, structure, stoichiometry, applied potential, time etc.) resulted in the proposal of different and partly conflicting models correlating compositional changes to electrode performance. In our study we utilize dense, geometrically well-defined microelectrodes which have been developed to a useful and well-established tool for basic studies in solid state electrochemistry. By reducing the systems complexity, it is possible to focus on a selected aspect, i.e. the influence of surface composition. Thin film perovskite microelectrodes with different compositional surface modifications represent model systems for surface enrichment by segregation. LSM (Lanthanum Strontium Manganese Oxide, mainly electronic conductive, currently used material in commercial SOFCs) and LSC (Lanthanum Strontium Cobalt Oxide, mixed ionic and electronic conductor, potential for intermediate temperature SOFCs) on Yttria-stabilized Zirconia (YSZ, poly and single crystalline) have been prepared and surface-covered with various oxides by PLD (pulsed laser deposition). For characterization we use SEM, (HR)XRD, AFM, and electrochemical impedance spectroscopy (EIS).Initial EIS results evaluating LSC microelectrodes on YSZpoly with a Gadalinium-doped Ceria (GDC) interlayer showed a different influence (activation or passivation) due to an increase of Sr, La or Mn content on the electrode surface. More detailed studies are ongoing. We like to report about the results of these investigation, present insights in surface compositions influence on ORR, and propose a model for explanation. These results will contribute to a better understanding of basic electrode processes for SOFC cathodes.
3:15 PM - GG5.4
Surface Properties and Durability of Electrochemically Grown Ultrathin Metal Catalysts.
Robert Rettew 1 , Faisal Alamgir 1
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractUltrathin metal films, consisting of less than 10 monolayers of one metal coating the surface of another, are currently of great interest for catalytic applications. These films are often fabricated electrochemically, resulting in structures such as core-shell particles or high surface area substrates in which the film provides a conformal coating over a porous support. This study correlates three important aspects of ultrathin metal films: (1) fundamental electron exchange mechanisms studied by x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS), (2) structure-dependent electrochemistry including fuel oxidation and desorption reactions, and (3) durability under reaction conditions as a function of layer thickness.By fabricating samples using surface limited redox replacement (SLRR), atomic monolayers and multilayers of Pt and other noble metals have been prepared electrochemically with accuracy at sub-monolayer coverages. This technique is currently of extreme interest due in part to the possibility of minimizing the usage of expensive noble shell metals such as Pt and Pd, but also because of the unique and advantageous activities and selectivities that can be obtained through the creation of a bimetallic catalyst with precisely controlled surface composition and structure. The type of overlayer-substrate samples we are creating are well suited to an exploration of electron tunneling and donation modes occurring in the near-surface regime of a bimetallic surface structure. By detecting thickness-dependent shifts in the binding energy and electron orbital occupancies of the overlayer material, we can infer the influence of the buried substrate metal on the electronic structure at the surface. This approach has allowed us to estimate the critical thickness above which a catalytically active ultrathin layer is entirely unaffected by the underlayer. Below this critical thickness, the surface catalytic properties can be tuned. For example, we have demonstrated significant shifts in electrochemical desorption curves as the Pt overlayer is grown from 0.3ML to 2ML. Finally, this study investigates the causes and mechanisms of rearrangement and corrosion for Pt overlayers on Au supports. During the oxidation of ethylene glycol or methanol on such Pt/Au systems, loss of Pt surface area has been observed with repeated cycling. This loss of surface area can be attributed to Pt metal coalescing into islands and subsequently being removed from the surface. By approaching the fundamental properties at an ultrathin interface from a traditional surface science standpoint as well as a practical reactivity and durability study, this work provides a comprehensive study of the performance as well as the electronic structure of ultrathin metal overlayers.
3:30 PM - GG5.5
Chemical, Electronic and Nanostructure Dynamics on Sr(Ti1-xFex)O3 Thin-film Surfaces at High Temperatures.
Yan Chen 1 , Woo Chul Jung 2 , Zhuhua Cai 1 , Helia Jalili 1 , Harry Tuller 2 , Bilge Yildiz 1
1 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractFor enabling durable and economic solid oxide fuel cells (SOFCs) at intermediate temperatures, highly active cathode surface chemistries are needed for oxygen reduction. For this purpose, the equilibrium surface structure, chemistry and electronic properties at the SOFC working conditions of temperature, pressure and potential need to be fully elucidated. The aim of our investigation is to correlate the structural, electron tunneling and chemical characteristics of cathode surfaces at the nanoscale and under in situ conditions to obtain a fundamental understanding of oxygen reduction at the atomistic level.The material we focus on in this study is Sr(Ti1-xFex)O3 (STF) in the form of dense thin films, grown by pulsed layer deposition (PLD) on single crystal yttria-stabilized zirconia (YSZ) substrates. The superior mixed-conductivity and the capability of controlling the conductivity through Fe doping render STF an attractive model cathode material for our study. In this work, high resolution probing of the surface structure and electronic state is achieved via Scanning Tunneling Microscopy and Spectroscopy (STM, STS) modified to enable high temperature and O2 environment during the experiments. Moreover, X-ray Photoelectron Spectroscopy (XPS) within the same chamber is used to obtain chemical information of the surface before and after the STM/STS measurements at controlled temperature and oxygen pressures. The surface of highly textured Sr(Ti0.65Fe0.35)O3 exhibited grains of size 20 to 30 nm, and was clearly enriched in the A site ion (Sr) in its as-prepared state, with an A site to B site ratio of 2.1 as compared to the bulk nominal value of 1. STS showed that the electronic structure transitioned from an insulating to metallic state from room temperature to 400oC, as opposed to the bulk semiconducting characteristics. At temperatures up to 630oC, the surface topography remained stable when in oxygen pressures of 10^(-4) mbar. When the sample is reduced by heating up to 700oC in 10^(-4) mbar oxygen and up to 630oC in 10^(-10) mbar, the surface Fe concentration at the B site increased to higher values than the bulk value of 35%, by an additional maximum value of 12%. Annealing at 630oC in reducing conditions of 10^(-10) mbar induced formation of particles on the surface, preferentially nucleating at the grain boundaries. Combining the results of STM, STS and XPS, the Fe-enrichment in reducing conditions on the surface may occur simultaneously in two possible structures: 1) formation of Fe-enriched phases as particles separating out of the STF film, and 2) the replacement of Ti by Fe while retaining the same surface phase. Investigation of the local electronic structure changes that occur upon the controlled temperature and oxygen pressure conditions are ongoing.
3:45 PM - GG5.6
Multidimensional Modeling of Thin Film Mixed Conductors: The Case of Ceria.
Francesco Ciucci 1 , William Chueh 2 , Yong Hao 2
1 Applied Mathematics, University of Heidelberg, Heidelberg Germany, 2 Materials Science And Engineering, Caltech, Pasadena, California, United States
Show AbstractIn order to design, optimize, and characterize solid oxide fuel cell (SOFC) electrodes, it is very useful to have models that aid in interpreting experimental results. samarium doped ceria (SDC) electrodes are currently of great interest for SOFC applications, three way catalysts and electrolyzers. For example, in SOFCs ceria-containing anodes can be operated directly on hydrocarbons without coking, and in addition can be used at lower temperatures than Ni/yttria-stabilized zirconia cermets due to their mixed conducting behavior under sufficiently reducing conditions. In this work, we first present a linear, time-dependent, multidimensional model for the study of SDC thin film samples at the continuum level. This model allows the computation of species concentrations, electric potentials and currents under small bias harmonic perturbations. A regular perturbation of the drift-diffusion equations and Poisson’s equation is used to derive the model for the electrochemical behavior of bulk of the material. The model includes:(1)The kinetics of reactions occurring at the SDC | gas surface (the SDC is exposed to a spatially uniform hydrogen-water-argon mixture at fixed total pressure). (2)In depth study of double layer corrections, necessary in the case of the electroneutral formulations [1].(3)Rigorous numerical error analysis and anisotropic mesh adaptivity.The response to small harmonic voltage inputs is computed at various hydrogen and water partial pressures. The numerical procedure allows for fast computations, for determination of fast and rate limiting steps and for direct numerical separation of various electrochemical contributions.We calculate impedance spectra and compare them to impedance experiments. The polarization resistance and chemical capacitances are calculated for a wide variety of cases. A brief explanation on how this model can be used to study cathode materials such as La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF), and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is included.References:[1] F. Ciucci, Y. Hao and D.G. Goodwin. Phys. Chem. Chem. Phys., 2009, DOI:10.1039/ B907740E
4:30 PM - GG5.7
First-principles Study on the Oxygen Diffusion in the La-apatite Oxide.
Ting Liao 1 , Taizo Sasaki 1 2 , Shigeru Suehara 1 , Ziqi Sun 3
1 Computational Materials Science Center, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 2 ICNSEE, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 3 WPI International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractThe lanthanum silicate apatite is an ionic conductor showing the oxygen migration. Nowadays it is regarded as one of the candidates of the material for the next generation solid oxide fuel cell. In spite of its high performance in the conduction property, the mechanism of the oxygen transport has not been established well yet. We have studied the behavior of the defects in La9.33Si6O26 with the first-principles theory within the density-functional theory. The present content of La assumes the existence of the La vacancies in the perfect apatite structure, which is expected to realize strong effects by the inhomogeneous charge distribution. In this study, the La vacancies have been explicitly configured in the model supercell, and the stable position of the oxygen interstitial and the migration path, which has been considered to dominate the transport, were determined. The results have shown that the excess oxygen ion forms the split interstitial pair with the original oxygen ion near the La vacancy as the stable geometry. The migration search ended in the results (1) that the barrier height was in the observed range, and (2) that the La vacancy plays an essential role in the barrier height by making the potential energy profile bumpy This research was partly supported by Grant-in-Aid for Scientific Research (C) (No. 22560663) from Japan Society for the Promotion of Science.
4:45 PM - GG5.8
First-principles Study of Defect Migration in RE-doped Ceria (RE = Pr, Gd).
Pratik Dholabhai 1 , James Adams 1 , Peter Crozier 1 , Renu Sharma 1 2
1 Materials Science & Engineering, Arizona State University, Tempe, Arizona, United States, 2 Center for Nanoscale Science & Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractOxygen vacancy formation and migration in ceria is central to its performance as an ionic conductor. Ceria doped with suitable aliovalent dopants enhances its oxygen ion conductivity, which is observed to be two-three orders of magnitude higher than yttria stabilized zirconia, the most widely used electrolyte material in solid oxide fuel cells. To understand the atomic defect migration in certain promising electrolyte materials for fuel cell applications, we present total energy calculations within the framework of density functional theory (DFT+U) to study oxygen vacancy migration in ceria, Pr-doped ceria (PDC) and Gd-doped ceria (GDC). We report activation energies for various oxygen vacancy migration pathways in PDC and GDC. Oxygen vacancy formation and migration were evaluated for first, second, and third nearest neighbor positions to a Pr and Gd ion. Results pertaining to the most preferred oxygen vacancy formation sites and most preferred migration pathways in these materials will be elaborated. Overall, the presence of Pr and Gd ions significantly affects oxygen vacancy formation and migration, in a complex manner requiring the investigation of many different migration events. We propose a relationship illuminating the role of additional dopants towards lowering the activation energy for vacancy migration in PDC and GDC. A comparison between calculated activation energies for oxygen vacancy migration in these materials and the reported measured and calculated values from literature will be discussed. This work has provided a foundation for the development of a kinetic lattice Monte Carlo model for vacancy diffusion in doped ceria, which will improve the understanding of oxygen ion conductivity in these materials.
5:00 PM - GG5.9
Pathways of Oxygen Incorporation at Pt/YSZ Interface.
Joong Sun Park 1 , Joon Hyung Shim 1 4 , Young Beom Kim 1 , Xu Tian 3 , Turgut Gur 2 , Fritz Prinz 1 2
1 Mechanical Engineering, Stanford University, Stanford, California, United States, 4 Mechanical Engineering, Korea University, Seoul Korea (the Republic of), 3 Applied Physics, Stanford University, Stanford, California, United States, 2 Materials and Science Engineering, Stanford University, Stanford, California, United States
Show AbstractDetailed understanding of the pathways for cathodic reduction of oxygen is of great interest for optimizing the performance of SOFC. And polarization losses associated with the cathodic reduction of oxygen make up a significant fraction of the total cell loss during SOFC operation especially at low temperatures. Electrode performance is strongly dependent on geometry of triple phase boundary (TPB) where the charge transfer reaction occurs. Many studies have proposed that TPB should be defined as a zone with a finite width rather than a singular line, and that the area of TPB depends on material properties, interfacial microstructure, operational conditions, and gas diffusion behavior [1-3].In this work, we report the results of a high spatial resolution spectroscopic study to gain insight and understanding of the oxygen surface kinetics and transport at the Pt/YSZ interface that may ultimately lead to advancements towards minimizing cathodic losses for intermediate temperature SOFCs. Pt was used and investigated as a cathode material since it is known to be the best catalyst for low temperature operation, giving suoperior performance [4-6]. In addition, its low solubility for oxygen makes it attractive to study TPB characteristics.For this purpose, we used oxygen isotope (18O2) to trace the diffusion of oxide ions near the TPB at the Pt/YSZ interface. The diffusion profile was obtained by NanoSIMS analysis. To prepare well-defined interfaces, dense sputtered platinum pads were fabricated by electron beam lithography on single crystal YSZ(100) (MTI Corp.). For high resolution profiling, CAMECA NanoSIMS 50L with 35nm Cs+ was used for SIMS analysis. Samples were annealed under 18O2 gas (>99%) at a pressure of 150torr with an applied cathodic bias (300mV to 2V) to reduce oxygen at the TPB at 300 -500°CFrom NanoSIMS results, we observed that 18O accumulated right next to THE Pt/YSZ interface indicating that oxygen reduction occurs at the TPB region. And the electrochemically active zone became wider and the relative ratio of 18O/16O at the interface increased with cathodic bias.To understand the relationship between 18O profiles and the applied bias, a continuum model have been developed to compare with experimental results. In the continuum model Poisson equation and the vacancy diffusion equations are solved iteratively to establish the electrical equipotential profiles inside the YSZ electrolyte. [1] T. Horita, K. Yamaji, N. Sakai, H. Yokokawa, T. Kawada, and T. Kato, Solid State Ionics., 127, 55 (2000). [2] J. Fleig, Annu. Rev. Mater. Res., 33, 361 (2003). [3] R. O’’Hayre, D. M. Barnett, and F. B. Prinz, J. Electrochem. Soc., 152 (2) A439 (2005). [4] H. Huang, M. Nakamura, P. Su, R. Fasching, Y. Saito, and F. B. Prinz, J. Electrochem. Soc., 154(1), B20 (2007).[5] J. Shim, C. Chao, H. Huang, and F. B. Prinz, Chem. Mater., 19, 3850 (2007). [6] P. Su, C. Chao, J. Shim, R. Fasching, and F. B. Prinz, Nano Lett., 8, 2289 (2008).
5:15 PM - GG5.10
Nonadiabatic Chemical-to-electrical Energy Conversion in Catalytic Schottky Junction Nanostructures.
Eduard Karpov 1 , Jyotsna Mohan 1
1 , University of Ilinois at Chicago, Chicago, Illinois, United States
Show AbstractNonadiabatic energy dissipation by electron subsystem of nanostructured solids unveil interesting opportunities for the solid-state energy conversion and sensor applications [1,2]. We found that planar Pt/GaP and Pd/GaP Schottky structures with nanometer thickness metallization demonstrates a nonadiabatic channel for the conversion into electricity the energy of a catalytic hydrogen-to-water oxidation process on the metal layer surface. The observed above thermal current greatly complements the usual thermionic emission current and its magnitude is linearly proportional to the rate of formation and desorption of product water molecules from the nanostructure surface. The possibilities of utilizing the nonadiabatic functionality in solid-state chemical-to-electrical energy conversion devices are discussed. Refs: [1] Karpov EG, Nedrygailov II. Nonadiabatic Chemical-to-Electrical Energy Conversion in Heterojunction Nanostructures. Physical Review B 81, 205443, 2010. [2] Karpov EG, Nedrygailov II. Solid-State Electric Generator Based on Chemically Induced Internal Electron Emission in Metal-Semiconductor Heterojunction Nanostructures. Applied Physics Letters 94, 214101, 2009.
5:30 PM - GG5.11
First-principles Studies of Formic Acid Oxidation on Pt Alloy Surfaces.
Liang Qi 1 , Ju Li 1
1 Dept. of Materials Science & Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractFormic acid (HCOOH) is a promising fuel candidate for next-generation fuel cell because of its high energy density, fast oxidation kinetics and convenience of usage as liquid state. In addition, the oxidation of formic acid on anode side is a generic model reaction to understand the mechanisms of electro-oxidation of small organic molecules for many other applications. Currently Pt is used as electrocatalyst for HCOOH oxidation. However, it suffers the problems of CO poisoning and high overpotentials, so several types of Pt alloys (PtRu, PtPb, PtBi, et al.,) are used in order to increase both CO tolerance and reaction rate at low anode potential. Here we use first-principle methods to study whole reaction paths of HCOOH oxidation, where the so-called “dual path” mechanisms are explored in details: free energy landscapes of the electrochemical HCOOH oxidation as a function of the electrode potential were obtained for each path (‘‘indirect’’ and ‘‘direct’’), and the activation barrier for each elementary step is approximately obtained based on nudged elastic band (NEB) calculations at solid/gas interfaces. Electronic structure analyses are also conducted in order to illustrate the intrinsic mechanisms of activity variations on different Pt alloy surfaces and benefit the design of new high-performance electro-catalysts.
5:45 PM - GG5.12
Using DFT Calculations to Understand Control of Direct Peroxide Synthesis Mechanisms on Transition Metal Catalysts in Electrochemical Environments.
Rees Rankin 1 , Jeffrey Greeley 1
1 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractDevelopment of highly efficient, specifically-functionalized and selective electrode catalysts is critical in bringing fuel cells and general ORR catalysts online for widespread true next-generation energy related technologies. In this work we present the results of first-principles based DFT calculations that characterize the selective reaction(s) of hydroxy-related surface species and intermediates on a set of transition metal based catalyst surfaces. Our work is generalized towards developing a broader effort in implementation of a materials-by-design computational database for catalytic reaction screening and design. In this specific presentation we will address interesting trends observed from the specific characterization of the fundamental mechanistic insights which may inhibit (or promote) the direct peroxide synthesis mechanism. Additionally, to extend our DFT calculations to the more appropriate fuel-cell like environment, the effects of electrochemical potential are considered on the reaction free energy and reaction selectivity. While our initial work focuses on analysis of ideal single component transition metal catalysts, we extend our study to further consider the effects of catalyst structure (ideal surface vs. stepped/defect surface). Additionally, we present results based on screening through likely preferential alloys, near-surface alloys, and skins of the most active transition metals identified.
GG6: Poster Session: Polymer-electrolyte Fuel Cells and Materials for Fuel Cells
Session Chairs
Byungwoo Park
Karen Swider-Lyons
Wednesday AM, December 01, 2010
Exhibition Hall D (Hynes)
9:00 PM - GG6.1
Platinum Multipods with Controllable Branch Length – Excellent Catalysts for Oxygen Reduction in Fuel Cells.
Yujie Xiong 1
1 School of Engineering & Applied Science, Washington University in St. Louis, St. Louis, Missouri, United States
Show AbstractPlatinum is the most effective catalyst to facilitate both hydrogen oxidation and oxygen reduction in a proton-exchange membrane (PEM) fuel cell. Controlling the morphology of platinum nanostructures can provide a great opportunity to improve their catalytic properties and increase their activity on a mass basis. Most recently, we have developed a facile synthetic approach to platinum multipods with controllable branch length. These platinum nanostructures exhibit relatively large surface area and particularly active facets which result in higher activity than the commercial Pt/C catalysts in oxygen reduction reaction, a rate-determining step in a PEM fuel cell. In particular, the controllable branch length enables to tune the catalytic activity of platinum nanostructures. It is expected that the present work will enable us to manipulate the catalytic performance of platinum nanostructures and provide a new platform for designing new fuel cell catalysts.
9:00 PM - GG6.10
Carbon Nanotubes Supports for Electrocatalytic Reduction of Oxygen with Cobalt and Iron Phthalocyanines and Porphyrins.
Adina Morozan 1 , Stephane Campidelli 2 , Bruno Jousselme 1 , Serge Palacin 1
1 Chemistry of Surfaces and Interfaces, CEA/DSM, Gif sur Yvette France, 2 Molecular Electronics Laboratory, CEA/DSM, Gif Sur Yvette France
Show AbstractElectrocatalysts play a significant role in the electrochemical systems for energy generation, such as proton exchange membrane fuel cell (PEMFC). The conventional electrocatalysts for the oxygen reduction reaction (ORR) are carbon-supported platinum-based materials, and may include other precious metals. An important goal of on-going research is to develop materials with high electroactivity, high efficiency and long term stability as alternative non-noble-metal catalysts for ORR, also with decreasing material cost. As an alternative cathode catalyst towards oxygen reduction, different iron or cobalt phthalocyanines (Pc) and porphyrins (P) has drawn much attention in this area. It has been demonstrated that the existence of specific interactions between the porphyrin or phthalocyanine and black carbon (BC) as well as the porphyrin/porphyrin play a crucial impact on the electrocatalytic O2 reduction especially on a 2e- or 4e- reduction mechanism pathway. In the other way, porphyrins or phthalocyanines are well know to stack on carbon nanotubes that present excellent electronic properties and are more and more used in materials electrodes. So, in this general context, different carbon nanotubes supports such as MWNTs (activated and non-activated), DWNTs, and SWNTs were investigated to replace the commonly Vulcan XC72R carbon support in Pc/BC or P/BC catalysts. The MPc or MP (M=Co, Fe) were dispersed homogeneously on the surface of CNTs using a solution process and inserted in a Nafion ink. Then, electrocatalytic properties and electrochemical stability of the catalysts were evaluated by Rotating Disk Electrochemistry measurements and cyclic voltammetry in acidic or basic conditions. The results demonstrate that CNTs are potential better support that Vulcan XC72R for porphyrins and phthalocyanines cathode materials for PEM fuel cells, with increased performance and stability.
9:00 PM - GG6.11
The PEMFC Using Stainless Steel Bipolar Plates Coated with Electrically Conductive and Corrosion Resistant CNT/PTFE Composite Film.
T. Nakashima 1 , Y. Fukami 1 , H. Murata 1 , S. Ishikawa 1 , T. Hisano 1 , D. Fukushiro 1 , R. Kuwabara 1 , T. Seimiya 1 , Yoshiyuki Show 1
1 Dept. of Electrical and Electronic Engineering, Tokai University, Hiratsuka, Kanagawa Japan
Show AbstractStainless steel bipolar plate for the polymer electrolyte membrane and the direct methanol fuel cells(FCs) has advantages of high manufacturability and mechanical strength. However, the stainless steel is corroded during operation of the fuel cell, because the inside of the fuel cells become acids condition. The most popular corrosion treatment is the coating of gold film by the electrochemical process. However, this treatment causes the fabrication cost of the bipolar plate to be high.In this paper, composite film was formed from the carbon nanotube (CNT) and the polytetrafluoroethylene (PTFE). This composite film is electrically conductive and highly corrosion resistance. The CNT/PTFE composite film was coated on the stainless bipolar plates as anticorrosion resistance coating. The bipolar plates were used for polymer electrolyte membrane fuel cell.The CNT/PTFE dispersion with the CNT concentration of 25% was applied to stainless steel bipolar plate at the thickness 50μm. The bipolar plates were dried under the atmosphere of 40oC for 30 min, and then were heated at 350oC for 10min. The CNT/PTFE film was uniformly coated by this process.Polymer exchange membrane fuel cells were assembled with either bare stainless steel bipolar plates or the bipolar plates coated with the CNT/PTFE composite film. The fuel cell using the bare stainless steel bipolar plates showed the output power of 2.0W. The composite film coating to the bipolar plates increased the output power up to 2.7W. Impedance analyzer measurement for these fuel cells indicated that the composite film coating decreased the contact resistance between the bipolar plate and the MEA, because the composite film prevents the bipolar plate surface from corroding. These results indicate that the CNT/PTFE composite film is useful as anticorrosion coating to bipolar plate of fuel cell.
9:00 PM - GG6.12
Gold Nanoparticle Enhancement for Polymer Electrolyte Membrane(PEM) Fuel Cell.
Cheng Pan 1 , Kenny Kao 2 , Sisi Qin 1 , Miriam Rafailovich 1
1 Materials Science & Engineering, Stony Brook University, Stony Brook, New York, United States, 2 Electrical Engineering, Stanford University, Stanford, California, United States
Show Abstract PEM fuel cell technology is one of the most promising future alternative energy sources because it has relatively low operating temperature, high power density, quick response, pollution-free operation. However, its relatively low power output compared to that of its price has prevented it from many practical applications. Gold nanoparticles have been widely known to possess special capabilities. Marvrikakis et al have predicted that gold nanoparticles that are platelet shaped and have direct contact to the substrate to be the “perfect” catalysts. In our experiment, hydrophobic, thiol-functionalized and spherical gold nanoparticles (around 2nm in diameter) were synthesized through two-phase method developed by Brust et al. When a solution containing these particles is spread at the air water interface, X-ray reflectivity and EXAFS spectroscopy indicate that some of the Au atoms are removed, as the water displaces the hydrophobic thiol chains from the particle surface, resulting in platelet shaped particles. Furthermore, after these nanoparticles are spread on the surface of water in a Langmuir-Blodgett (LB) trough where surface pressure can be applied to compress them, they form LB film consisting of one or more monolayers. This LB film can then be deposited onto a solid surface, such as the Nafion® membrane where the particle surface can make direct contact with electrodes and hydrogen/oxygen gases, and take effect. A series of parallel experiments were done to explore the various factors that may affect the performance of PEM fuel cell, such as hydrogen flow rate, surface pressure of the LB trough etc. We found that under the optimal hydrogen flow rate of 40 ccm (cubic centimeter per minute), gold nanoparticles resulted in more than 80% increase in the power output of the fuel cell. In order to figure out the exact role the gold nanoparticles play in this huge enhancement, we did electrochemical experiments (such as Cyclic Voltammetry, Oxygen Reduction Reaction, etc) on our gold nanoparticles and use carbon dioxide instead of oxygen during the fuel cell operation. We also find that there is an optimal surface pressure of the LB trough to create the highest enhancement of output power. To explain this, X-ray reflectivity measurement was done to get useful information for surface condition of gold nanoparticles on the Nafion® membrane.
9:00 PM - GG6.13
A Study of Effects of Composition, Lattice, and Size Parameters of PtIrCo and PtNiZr Electrocatalysts on Oxygen Reduction Reaction.
Rameshwori Loukrakpam 1 , Jin Luo 1 , Bridgid Wanjala 1 , Bin Fang 1 , Chuan-Jian Zhong 1
1 Department of Chemistry , State University of New York at Binghamton, Binghamton, New York, United States
Show AbstractThe ability to nanoengineer the activity and stability of multimetallic alloys is important for the design of advanced fuel cell catalysts. This presentation describes the results of an investigation of the synthesis of nanoparticles with two special trimetallic combinations, PtIrCo and PtNiZr, and the preparation of carbon-supported PtIrCo and PtNiZr electrocatalysts for oxygen reduction reaction. The trimetallic nanoparticles with controllable composition and size (1-10 nm) were synthesized by nanoengineered synthesis and processing methods. Carbon-supported trimetallic nanoparticles were thermally treated in the temperature region 400-900 0C under different reactive and non-reactive environments. The nanostructured surface and phase properties were characterized suing a wide range of characterization techniques including TEM, DCP-AES/ICP-MS, HRTEM, XRD, EDS, XPS, XAFS, etc. These catalysts were shown to exhibit enhanced electrocatalytic activity and stability for oxygen reduction reaction depending on a combination of the composition, lattice, and size parameters. The effect of temperature on the lattice parameter and its correlation with the activity and stability of the catalysts will be discussed.
9:00 PM - GG6.14
Zinc-gallium Oxynitrides as Visible-light Photocatalysts: Electronic Properties and Formation Energies.
Heather Schmidt 1 , Douglas Doren 1
1 Chemistry and Biochemistry, University of Delaware, Newark, Delaware, United States
Show AbstractSolid solutions of GaN and ZnO have been shown to be a promising class of photocatalysts, capable of splitting water under visible-light irradiation. The structural and electronic properties of Ga1-xZnxN1-xOx in the wurtzite structure have been studied using density-functional theory with the Linear Augmented Plane Wave (LAPW) method at varying values of x. A GGA+U approach is used to better describe the semicore 3d states of Ga and Zn. These calculations show that there exists a p-d coupling between the N 2p and Zn 3d states, leading to a decreased band gap. The band gaps in the mixed metal oxynitrides are lower than either ZnO or GaN, thus allowing excitation by visible light. The trend in band gaps over the range of Zn concentrations (x) is consistent with experimental results. However, the expected band gap minimum occurs at a composition with a high concentration of zinc that is difficult to synthesize. To understand the limitations on synthesis, formation energies have been calculated for various starting materials and synthesis environments to determine whether thermodynamics places a limit on the concentrations of zinc that can be obtained in these materials. Recent experimental work has demonstrated that a novel ZnGa oxynitride in the spinel structure can be produced from a ZnGa2O4 precursor. This material has high Zn content and low N content and is also photocatalytically active under visible light. The structural properties, electronic properties and energetics of this material will also be described.
9:00 PM - GG6.15
Nitrogen-containing Cobalt Complexes as Non-precious Catalysts for Polymer Electrolyte Fuel Cell.
Chen-Hao Wang 1 , Sun-Tang Chang 2 , Hsin-Cheng Hsu 2 , He-Yun Du 2 , Kuei-Hsien Chen 3 1 , Li-Chyong Chen 1
1 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, 2 Department of Chemical Engineering, National Taiwan University, Taipei Taiwan, 3 Institute of Atomic and Molecular Sciences , Academia Sinica, Taipei Taiwan
Show AbstractNon-precious metal catalysts, specifically, nitrogen-containing cobalt complexes, have been developed for polymer electrolyte fuel cell (PEFC). At present time, the most popularly used electro-catalysts for PEFC contain platinum catalysts that are supported by carbon blacks. Usually, the oxygen reduction reaction (ORR) in the cathode is much slower than the hydrogen oxidation reaction in the anode, and therefore, a high loading of Pt/C is used in the cathode to enhance the ORR, which makes the PEFC expensive. Alternative catalysts that are efficient, durable and most importantly, inexpensive, are highly desirable to make PEFC viable. Bashyam et al. had used cobalt-polypyrrole composite in the cathode in an H2-O2 PEFC and demonstrated a maximum power density of 150 mW cm-2 at 80 oC [1]. Lefevre et al. reported microporous carbon supported iron-based catalysts with active sites coordinated by pyridinic nitrogen functionalities showing performance relative to that of precious metals [2]. However, all these alternatives are still inadequate for the ORR in term of their activity and stability [3]. Here, high-performance non-precious catalysts using nitrogen-containing cobalt complexes supported on carbon blacks (Co-N-complex/C) will be presented. After simple pyrolysis, the Co-N-complex/C demonstrates an initial electron transfer number of 3.85 in ORR, which is very close to the ideal value of 4.0, and maintains at 3.75 after 30,000 cycles in potential cycling within 0.6 and 1.1 V. A power density of 316 mW cm-2 and a current density of 450 mA cm-2 at 0.5 V (or 1.2 A cm-2 at 0.5 ViR-free) has been achieved utilizing the Co-N-complex/C as the cathode catalyst in PEFC, which surpasses the performance of all the reported non-precious metal catalysts in the literature. Detailed studies including micro-Raman, FTIR, XPS, XANES, and EXAFS have been applied to unveil the transformation during the pyrolysis.Keywords: fuel cell, oxygen reduction reaction, non-precious catalyst, cobalt complexReferences:1.Bashyam, R.; Zelenay, P. Nature 2006, 443, (7107), 63-66.2.Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Science 2009, 324, (5923), 71-74.3.Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Science 2007, 315, (5809), 220-222.
9:00 PM - GG6.16
Structural Stability of AlH3 as Hydrogen Storage Material.
Masahiko Katagiri 1 , Shigeki Saito 1 , Hiroshi Ogawa 1
1 Computational Materials Science Center, National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractHydrogen is an effective fuel to overcome the environmental problem. However, hydrogen gas has very small energy per volume and is desired to develop effective way to dense and store hydrogen in a light material. For the purpose of Fuel-Cell Vehicle (FCV), the gravimetric hydrogen content is to be larger than 6 wt%. Aluminum hydride, AlH3, is promising as a hydrogen storage material due to its large gravimetric and volumetric content (10.1 wt% and 148 kg/m3, respectively). Despite very low hydrogen solubility and the fact aluminum is one of the most unlikely candidates to form a metal hydride, AlH3 can be synthesized in even different polymorphic structures by simple organometallic methods. Recently, a new direct-reaction method of synthesizing AlH3 has been developed under high temperature and pressure. If we succeed in synthesizing AlH3 or its alloys under moderate condition, AlH3 will be a hopeful hydrogen storage material. On the other hand, it is a problem that the hydrogenation stops near the interface region between Al and its oxide layers even at high pressure of 10GPa. We discuss the mechanism by slow diffusion of hydrogen using ab initio molecular dynamics (MD). Radial Distribution Functions (RDFs) for α-AlH3 crystal are calculated with experimental lattice constants. At elevated temperatures, H-H and H-Al peaks show broadening. However, the H-hoping are not observed. We also calculated phonon dispersion and could not find any imaginary mode. It was found that the structure is dynamically stable. The hydrogen ordering in Al lattice suppresses the H diffusion and the hydrogenation cannot be developed in whole material. We also discuss the bonding feature in AlH3.
9:00 PM - GG6.17
Preferentially Oriented (100) Platinum Electrodeposits with Enhanced Electrocatalytic Properties for Fuel Cell Applications.
Sebastien Garbarino 1 , Alexandre Ponrouch 1 , Erwan Bertin 1 , Daniel Guay 1
1 , INRS, Varennes, Quebec, Canada
Show AbstractAs reducing the loading of expensive catalysts and improving their specific activity both remain an extensive area of current fuel cell research, the synthesis of 1D metallic nanostructures was recently identified as a promising avenue to achieve this goal. In recent years, the use of 1D nanostructures attracted significant interest as a smart combination of high specific surface area catalysts (similar range order as nanoparticles on carbon support) with peculiar properties imposed by the anisotropy of their 1D geometry. Indeed, 1D nanostructure combines the advantage of high specific surface area nanoparticles with enhanced mass and electrical transport properties, as well as resistance to corrosion and sintering of nanoparticles.In the present work, Pt/Ti 1D nanostructures have been prepared by electrodeposition at constant potential through commercial porous AAO (Anodic Aluminum Oxides) membranes. The single step electrodeposition procedure was carried out in a solution containing Pt chloride salt. Following the potentiostatic deposition, the AAO membrane was dissolved by immersion in alkaline solution and the properties of the resulting material were examined. Full control of the deposition parameters (electrode potential, time, temperature, concentration, supporting electrolyte, etc ...) lead to Pt nanostructures with different aspect ratio and morphology, as well as preferential orientation in the bulk and at the surface of the electrodeposits. As will be demonstrated by X-ray diffraction (XRD) measurements and confirmed by cyclic voltammetries (CV) in sulphuric acid, the total proportion of (100) site is controlled by the deposition parameters. Thus, the (200)/(220) peak intensity ratio of the XRD pattern, which indicates a preferential bulk orientation, and the (H100/H110) hydrogen desorption peaks ratio from the CVs, which indicates that a preferential (100) surface orientation exists, were both found to vary in a similar way with the deposition parameters. In the case of the more oriented surfaces, a significant increase of the electrocatalytic activity towards many surface sensitive reactions like NH3 and N2H4 were observed compared to none-oriented Pt surface of identical roughness (ca. 150). Such control on the crystallographic surface orientation of platinum, combined with large electrochemically active surface areas, are two key issues for surface sensitive reactions.
9:00 PM - GG6.18
Crosslinked Sulfonated Polystyrene Nanofiber Mats for Ion Exchange.
Chitrabala Subramanian 1 , R. Weiss 2 1 , Montgomery Shaw 1
1 Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 2 Department of Polymer Engineering, The University of Akron, Akron, Ohio, United States
Show AbstractSulfonated polymers, either as a pure polymer or as blends have been explored for making proton exchange membranes (PEMs) for fuel cells, among other applications. In this work we successfully electrospun highly sulfonated polystyrene (SPS) nanofibers for use in composite PEMs and studied the process of electrospinning this difficult-to-spin material. For application in membranes, the high degree of sulfonation (~4.5 meq/g) helps attain practical ion exchange capacities for the membrane; however, the fibers become very water soluble. One limitation with using these fiber mats is to strike a balance between proton conductivity and water stability at high sulfonation levels. The high degree of sulfonation of these polymers gives us the flexibility to sacrifice some of the sulfonic acid groups by utilizing them for crosslinking without lowering the proton conductivity of the composite membrane greatly. We explored the possibility of adding polyethylene glycol (PEG) 400 to the polymer solution and subjecting the cast film to thermal treatment. The hypothesis is polycondensation reaction between hydroxyl groups end groups of the PEG and sulfonic acid groups from SPS leads to crosslinking. These heat-treated films showed higher Tg than neat SPS films and the water solubility of the membranes dropped. The crosslinked films were swollen in D2O and both the swollen polymer and polymer leached in D2O were analyzed by NMR. The leached out crosslinked films did not show any sulfonation peaks while the crosslinked swollen portion did, confirming that the heat treatment reduced the solubility of sulfonated polymer and tendency to leach out in water. We also attempted using high molecular weight polyethylene oxide (100 kDa). Addition of PEO to the polymer solution helped improve the electrospinning process of the highly sulfonated polymer and the heat treatment improved stability of the fiber mats in water. Our goal is to understand the chemistry of this crosslinking reaction and use the material to fabricate efficient ion-exchange membranes.
9:00 PM - GG6.19
Carbon Nanotube Supported g-C3N4 Catalyst for Oxygen Reduction Reaction in Acidic Media.
Hyun-Jong Kim 1 , Ji-Eun Ahn 1 , Kyung Ryun Kang 1 , Myung-Keun Han 1
1 Surface Technology Center, Korea Institute of Industrial Technology (KITECH), Incheon Korea (the Republic of)
Show AbstractThe cathodic oxygen reduction reaction (ORR) is one of the most crucial factors in the performance of polymer electrolyte membrane fuel cell (PEMFC). The development of efficient ORR electrocatalysts is thus of great significance for the commercialization of fuel cells. Platinum based materials have long been investigated as active catalysts for ORR. However, the large-scale application of fuel cells has been hampered by the high cost and inadequacy of this metal. Recently, nonprecious-metal and metal-free catalysts for ORR have attracted enormous interest as an alternative to platinum based catalysts. In particular, nitrogen-doped carbon materials, which are typical metal-free catalysts, exhibit excellent electrocatalytic activity for ORR as a result of their unique electronic properties derived from the conjugation between the nitrogen long-pair electrons and the graphene π system.In this study, we thermally synthesized g-C3N4/CNT as non-platinum catalysts for ORR catalyst. The key properties of CNT for electrochemical devices are good electrical conductivity and large surface area with high porosity. The g-C3N4/CNT catalyst was characterized by FT-IR and XRD. For the material condensed at 823K, an in-planar repeat period 0.681nm in the crystal is evident from the XRD pattern, which is smaller than one tri-s-triazine unit, presumably owing to the presence of small tilt angularity in the structure. The strongest XRD peak at 27.4o is a characteristic interplanar stacking peak of the conjugated aromatic systems, indexed for graphitic materials as the (002) peak. It should be noted that all of the materials feature some residual amount of hydrogen, which decrease with increasing condensation temperature. The FT-IR absorption spectrum observed at 1310 and 1610cm-1 correspond to the C-N in the aromatic/conjugated cycles. The peak at 830cm-1 is attributed to the vibration perpendicular to the aromatic/conjugated cycles. The band observed at 3200cm-1 could be due to the N-H bonds. The elecrocatalytic oxygen-reduction reaction (ORR) measured by using rotating disk electrodes (RDE) for C3N4 and C3N4/CNT catalysts. An onset potential of C3N4 was approximately 0.6V (RHE) and current density was 1.8mA/cm2. On the other hand, an onset potential of C3N4/CNT catalyst was approximately 0.64 V (RHE) and current density was 4.5mA/cm2. The C3N4/CNT shows good catalytic activity among the samples cathode catalyst. The ORR result of C3N4 catalyst showed after different heat treatment. As increasing the thermal treatment up to 600oC the catalytic activity was strikingly improved. After 700oC, however, the materials were fully decomposed. Above 700oC, the material vanishes without residue under the formation of highly reactive nitride species, which allows efficient nitrification of diverse metal oxides under relatively mild condition.
9:00 PM - GG6.2
Electrochemical and Electrical Properties of Carbon-free Pt Electrocatalysts Supported on Doped SnO2 for Polymer Electrolyte Fuel Cells.
Fumiaki Takasaki 1 , Zhiyun Noda 2 , Yusuke Shiratori 1 , Kohei Ito 1 2 , Kazunari Sasaki 1 2
1 Faculty of Engineering, Kyushu University, Fukuoka Japan, 2 International Research Center for Hydrogen Energy, Kyushu University, Fukuoka Japan
Show AbstractCarbon support corrosion especially in cathode electrocatalysts for polymer electrolyte fuel cells (PEFCs) is one of the important technological issues to be solved. We focus our attention to SnO2, known as a broad-band oxide semiconductor with a high electronic conductivity, as a possible electrocatalyst support material. In this study, nanostructures, electrical conductivity of support materials, electrochemical properties and the durability of carbon-free electrocatalysts are investigated, including Pt/SnO2, Pt/Nb-SnO2, Pt/Al-SnO2. The oxide support materials were prepared by the co-precipitation procedure, and Pt nano-particles were impregnated on support materials via a colloidal method. According to the FE-SEM observation, Pt catalyst particles have been prepared homogeneously with a diameter of ca. 3 nm. The electrochemical surface area (ECSA) was determined from the hydrogen desorption region obtained in the CV measurement. Pt/Sn0.98Nb0.02O2 electrocatalyst exhibits a large ECSA, which is comparable to that of conventional Pt/C prepared by the same procedure, because of high electrical conductivity and high surface area of doped SnO2. These electrocatalysts based on SnO2 as the support material have a considerable tolerance against cycles of voltages between 0.6 and 1.3 VRHE. Even after 10,000 times of voltage cycles, these carbon-free electrocatalysts still have sufficient ECSAs. These results indicate that the use of SnO2-based carbon-free electrocatalysts can improve long-term durability of PEFCs against severe voltage cycling, simulating the typical start-up situations of automotive PEFC systems.
9:00 PM - GG6.21
Catalytical Mechanism of Nitrogen-doped Graphene for Efficient Catalytical Electrodes in Fuel Cells.
Lipeng Zhang 1 , Zhenhai Xia 1 , Liming Dai 2
1 Department of Mechanical Engineering, The University of Akron, Akron, Ohio, United States, 2 Department of Chemical Engineering, Case Western Reserves University, Cleveland, Ohio, United States
Show AbstractThe rising global energy demand and the environmental impact of energy use from traditional sources pose serious challenges to human health, energy security, and environmental protection. One of the promising fields of clean and sustainable power is fuel cell technology, based on direct conversation of fuel into electricity. However, fuel cells need precious metals – primarily Pt – as catalyst counter electrode to promote the chemical reaction that generates power. The limited resources and high cost of the platinum catalysts has been shown to be the major "showstopper" to mass market fuel cells for commercial applications. Efforts are needed to search an alternative material which is readily available, cost effective and can show comparable catalytic effects for catalytic cathodic reduction in fuel cells. Recent experiment shows that nitrogen-doped graphene is an excellent catalyst for fuel cell electrodes, promising to replace the expensive platinum electrodes. In this study, first-principle simulations were performed to investigate the catalytical mechanisms of the N-doped graphene. We show that the improved catalytic performance is attributed to the electron-accepting ability of the nitrogen atoms, which creates net positive charge on adjacent carbon atoms in the carbon plane. These “N-doping complex” readily attract electrons from the anode for facilitating the four-electron oxygen reduction, which is efficient catalytical route. The various structures of “N-doping complex” as catalytical sites are examined in terms of efficiency for catalyzing oxygen reaction reduction on the electrodes in fuel cells.
9:00 PM - GG6.22
High Temperature Crystal Structure and Electrochemical Properties of RBa(Co,Zn)4O7±δ.
Jung-Hyun Kim 1 , Young Nam Kim 2 , Zhonghe Bi 3 , Ashfia Huq 1 , M. Parans Paranthaman 3 , Arumugam Manthiram 2
1 Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Materials Science and Engineering, University of Texas at Austin, Austin, Texas, United States, 3 Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractTransition metal oxides with mixed oxide-ionic and electronic conducting (MIEC) properties find unique applications as electrode (cathode and anode) materials in SOFC and as oxygen separation membranes. In particular, cobalt-based oxides such as RBaCo4-xMxO7±δ (R = Y, Ca, In, Lu, Yb, Tm, Er, Ho, and Dy and M = Co, Zn, Fe, Al, and Ga) consisting of corner-shared (Co,M)O4 tetrahedra has attracted interest as a potential oxygen storage material in addition to exhibiting interesting properties as a cathode for solid oxide fuel cells (SOFC). For example, RBaCo4-xZnxO7±δ exhibited low TECs of 6 × 10-6 – 13 × 10-6 oC-1 in a range of 80 – 900 oC, which provides a good thermal expansion compatibility with standard SOFC electrolyte materials. In addition, YBaCo4-xZnxO7 + Gd0.2Ce0.8O1.9 (GDC) composite exhibited good cathode performance comparable with that obtained with the cobalt-based perovskite cathodes. This presentation will focus on the high temperature crystal structure and electrochemical properties of the RBaCo4-xZnxO7±δ. In particular, the in-situ powder neutron diffraction and structure refinement will be employed to investigate the crystal structure at high temperatures. In addition, influence of the R elements on the high temperature properties including phase stabilities, oxygen stoichiometry, and transport properties of the RBaCo4-xZnxO7±δ sample will be presented. Finally, electrochemical performances of the RBaCo4-xZnxO7±δ as cathode materials in SOFC will also be presented.Research sponsored by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (DMSE), ORISE, and ORNL’s LDRD program, under contract DE-AC05-00OR22725 with UT-Battelle, LLC managing contractor for Oak Ridge National Laboratory.
9:00 PM - GG6.23
Kinetic Lattice Monte Carlo Model for Oxygen Vacancy Diffusion in Doped Ceria: Applications to Materials Design.
James Adams 1 , Pratik Dholabhai 1 , Shahriar Anwar 1 , Peter Crozier 1 , Renu Sharma 1 2
1 Materials Science & Engineering, Arizona State University, Tempe, Arizona, United States, 2 Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractCeria related materials are considered as one of the most promising materials for fuel cell applications because of their high ionic conductivity, which in turn facilitates the reduction of their operating temperature and thereby eliminates several technological problems. Kinetic Lattice Monte Carlo (KLMC) methods have proven very useful in the investigation of oxygen diffusion in oxides. Here we apply the KLMC model to oxygen diffusion in doped ceria to investigate the effects of dopant concentration and temperature on ionic conductivity in these materials. The current approach uses a database of activation energies for oxygen vacancy migration, calculated using first-principles (DFT+U), for various migration pathways in Pr-doped ceria (PDC) and Gd-doped ceria (GDC). Also, since our first-principles calculations revealed significant vacancy-vacancy repulsion, we investigate the importance of that effect by conducting simulations with and without a repulsive interaction. The rationale behind the calculated distinct maximum in ionic conductivity as a function of dopant concentration derived using the two separate models, vacancy repelling and vacancy non-repelling models, will be discussed. Comparison with experimental measurements for ionic conductivity and average activation energy as a function of dopant concentration will be presented. Based on the agreement with experimental measurements, we believe that the current methodology comprising a blend of first-principle calculations and KLMC techniques can be used as a design tool to predict the optimal dopant concentration for attaining maximum ionic conductivity in these materials.
9:00 PM - GG6.24
Development of Composite Nanocatalysts and Their Enhanced Electrocatalytic Activity.
Youngmin Lee 1
1 , Brown University, Providence, Rhode Island, United States
Show AbstractMonodispersed Au, dumbbell-like Au-Fe3O4 and fcc-, fct-FePt nanoparticles were synthesized via solution phase synthesis and were used for electrocatalytic applications. Fcc-, fct-FePt nanoparticles are active as cathode catalyst for fuel cells in acidic media, where fct-FePt structure shows higher activity and stability than fcc-FePt towards oxygen reduction reaction in H2SO4 due to the distinct intermetallic nature. Au, Au-Fe3O4 nanoparticles are active as cathode catalyst in alkaline media, where Au shows higher activity than Au-Fe3O4 towards oxygen reduction reaction in KOH. Meanwhile, Au-Fe3O4 was found to be highly active for hydrogen peroxide reduction in neutral PBS solution, which showed higher activity and stability than its individual components Au or Fe3O4 that was directly obtained from Au-Fe3O4 by selective etching. The enhanced catalytic properties are due to a synergetic effect coming from the interconnecting area.
9:00 PM - GG6.25
Origins of Rapid Aging of Ba-based Proton Conducting Perovskites.
Aneta Slodczyk 1 , Bogdan Dabrowski 2 , Natalie Malikova 3 , Philippe Colomban 1
1 , LADIR UMR 7075 CNRS-UPMC, Thiais France, 2 Department of Physics, Northern Illinois University, DeKalb, Illinois, United States, 3 , LLB CEA-CNRS, Saclay France
Show AbstractProton conducting ceramics are considered as promising membranes for medium temperature fuel cells, water stream electrolyses and CO2/syngas converters. Materials for these applications have to be mechanically and chemically stable at extremely corrosive conditions of high water vapor pressure in order to ensure the long life-time (~10000 hours) operation. Several A2+(Sr,Ba)B4+(Zr,Ce,Ti,Sn,In,Ta,Nb)O3 perovskites modified by Rare Earths RE3+ or Lanthanides Ln3+ may possibly satisfy these demands. Our comprehensive Raman, infrared, thermogravimetric and thermal expansion studies have shown that the choice of A and B elements as well as the material processing (synthesis, geometry, density, etc.) are crucial to control aging of material. We will consider an example of Ba(Zr,Sn,In)O3 perovskites to show that several factors such as the carbonatation, the presence of parasite phases AO at the grain boundaries as well as the usage of samples with highly active surface, i.e. powders or lightly densified ceramics can cause: i) preferential adsorption of surface protonic species such as hydroxides, (hydro)carbonates, etc., ii) decreased incorporation of bulk protonic species responsible for the proton conduction, iii) significant modification of the symmetry/structure of the host perovskite structure up to complete decomposition of the material. We will show how to improve an industrial application of perovskites by understanding and controlling these processes. Acknowledgments:Work at NIU was supported by the NSF-DMR-0706610
9:00 PM - GG6.26
Synthesis, Material and Electrochemical Characterization of Cobalt-doped Barium Cerate-zirconate with Varying Zirconium and Cobalt Content.
Aravind Suresh 1 2 , Joysurya Basu 1 2 , Barry Carter 1 2 , Benjamin Wilhite 1 2
1 Chemical Engineering, University of Connecticut, Storrs, Connecticut, United States, 2 Center for Clean Energy Engineering, University of Connecticut, Storrs, Connecticut, United States
Show AbstractMixed protonic-electronic conducting ceramics are promising alternatives to palladium-based thin films for the production of high-purity hydrogen and as electrodes in intermediate-temperature solid oxide fuel cells. Acceptor-doped barium cerates and cerate-zirconate blends have been widely investigated as protonic and mixed protonic-electronic conductors [1, 2]. Our research group is investigating transition metal dopants for barium cerate-zirconate in order to incorporate catalytic activity for hydrogen generation in addition to mixed protonic-electronic conductivity for hydrogen purification Cobalt-doped barium cerate-zirconate (BCZC) is being investigated as a bi-functional electro-ceramic for catalytic hydrogen generation and subsequent purification from methanol by mixed protonic-electronic conductivity [3]. Material and electrochemical properties of the oxide are expected to depend on the cobalt doping and the A:B site ratios. Powders with nominal compositions BaCe0.25Zr0.75-xCoxO3-δ (x = 0.00 to 0.15, A:B ratio~1) were synthesized by solid state reaction. Phase and purity of the powders were examined using X-ray diffraction and electron microscopy. Electrical conductivity of the materials was studied using electrochemical impedance spectroscopy (EIS) conducted on sintered disks. Powders with nominal compositions BaCe0.25Zr0.75-xCo0.15O3-δ (x = 0.20 to 0.30, A:B ratio>1) were also investigated to understand the impact of A:B ratio on the characteristics of the material with the cobalt content fixed. [1] H. Iwahara, Solid State Ionics 77, 289 (1995)[2] J. Guan, S.E. Dorris, U. Balachandran, M. Liu, Solid State Ionics 100, 45 (1997)[3] A. Suresh, J. Basu, C.B. Carter, N. Sammes, B.A. Wilhite, J. Mat. Sci. 45, 3215 (2010)
9:00 PM - GG6.27
Optical and Defect Properties of Pr Doped Ceria.
Jae Jin Kim 1 , Sean Bishop 1 , Woo Chul Jung 1 , Harry Tuller 1
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMixed ionic and electronic conducting (MIEC) oxides play an important role in applications such as solid oxide fuel cells (SOFCs) and oxygen separation membranes. MIEC electrodes in SOFCs, for example, utilize the entire electrode surface for the reduction or oxidation of oxygen as opposed to only the triple phase boundaries while oxygen separation membranes using MIECs allow for a single phase material to provide simultaneously for oxygen diffusion and the counter-flow of electrons. In this presentation, Pr doped CeO2 (PCO) is investigated, given the ability to introduce MIEC in ceria (an important solid electrolyte with enhanced electrode kinetics) even at elevated pO2’s (e.g. air) resulting for the ready reduction of Pr to its trivalent state. Given the expected importance of the energetic positions of the Pr energy states in the ceria band gap, and their relevance to the defect structure, the wavelength dependent optical absorptivity of PCO is examined as a function of [Pr] and thermal history conditions (T, pO2) and correlated with electrical and oxygen stoichiometry measurements.
9:00 PM - GG6.28
High Surface Area Boron-doped Diamond Powders: New Corrosion-resistant Electrocatalyst Supports.
Doo Young Kim 1 , Liang Guo 1 , Vernon Swope 1 , Ayten Ay 1 , Greg Swain 1
1 Chemistry, Michigan State University, East Lansing, Michigan, United States
Show AbstractElectrically-conducting diamond powder (>100 m2/g, >0.5 S/cm) is gaining interest as an advanced electrocatalyst support for polymer electrolyte membrane (PEM) fuel cells due to strong electrocatalyst-support interactions, high surface area (> 100 m2/g), and superb electrochemical corrosion resistance. One factor limiting the performance of PEM fuel cells at present is degradation of the electrocatalyst layer, in particular, corrosion of the carbon support. During extended operation with start-stop cycling and fuel starvation, the cathode can experience potentials approaching 1.5 V vs. RHE. At this potential, the rate of carbon electrochemical corrosion is high according to the reaction: C + H2O → CO2 + 4H+ + 4e- (Eo = 0.207 V vs. NHE). Electrochemical corrosion of the support is cumulatively catastrophic causing catalyst particle agglomeration, increases the ohmic resistance through the powder network, and increases the gas diffusion resistance due to collapse of the porous structure. Clearly, support materials with improved stability and durability are needed for PEMFC operation, and diamond is a promising candidate to replace carbon black as the catalyst support. Our group is working on the preparation and characterization of high surface area electrically-conductive diamond powders. The approach involves overcoating (i) diamond powder (250 m2/g surface area), (ii) Ketjen black (800 m2/g) or (iii) TiO2 (220 m2/g) with a thin layer of boron-doped ultrananocrystalline diamond (B-UNCD). For diamond-coated powders chemically impregnated with electrocatalyst (e.g., B-UNCD on diamond), a uniform distribution of Pt particles is generally observed over the entire powder surface. The nominal particle size is 2-10 nm with the surface chemistry of the diamond powder having little effect on the electrocatalyst particle size or distribution. Pt particles form epitaxially with the diamond (111) planes. Defects develop in the Pt particles as a mechanism for stress relief due to the 10 % lattice mismatch between the two materials. Strong metal-support interaction (SMSI) exists as XPS data revealed a 1-1.2 eV higher binding energy shift of the Pt 4f7/2 and 4f5/2 photoelectrons for the metal supported on diamond as compared to the bulk metal. The Pt particles directly contact the diamond as EELS revealed no interfacial layer. The Pt-loaded diamond is stable as TGA, used as an accelerated degradation test, indicated negligible gasification below 500 oC in air, while equivalently loaded Vulcan XC-72 underwent significant gasification.
9:00 PM - GG6.29
Microwave-assisted Synthesis of Sulfonic Acid-functionalized Benzene-bridged Mesoporous Silica.
Eddy Domingues 1 , Paula Ferreira 1 , Filipe Figueiredo 1
1 , CICECO-University of Aveiro, Aveiro Portugal
Show AbstractProton exchange membranes based fuel cells (PEMFC) are nowadays sufficiently mature technologies. However, the properties of the commonly used membrane material - NAFION - sets the operation at low temperature (T<80°C) and saturated humidity. In these operation conditions, the electrode kinetics is poor, whereas the poisoning of the catalyst by CO may also occur, when using methanol or low purity H2 fuels, both factors implying important loads of the Pt catalyst. Moreover, the efficiency of the cell is decreased due to the electroosmotic drag of water molecules and the cross-diffusion of water or alcohol. The operation of PEMFCs at about 120-180°C is thus desirable. However, the scarce membrane materials, partly meeting the requirements for this temperature range, have been prompting the research for new intermediate temperature proton conductors. The potential of acid functionalized periodic mesoporous organosilicas (ac-PMO) has been recognized, either as a single-phase or as an adduct to NAFION. The functional acid groups as well as the organic-inorganic bridged precursors may be tuned to facilitate the proton structural diffusion via the ordered structures with meso- or molecular-scale periodicity. The sulfonic acid-functionalized benzene-bridged PMOs are usually synthesized by hydrothermal co-condensation of 1,4-bis(triethoxysilyl)benzene, (3-mercaptopropyl)-trimethoxysilane and the cationic surfactant template octadecyltrimethylammonium chloride in basic medium, followed by surfactant extraction and oxidation of thiol group to sulfonic acid group by reaction with HNO3. This synthesis is very time consuming (ca. 66 h) and the shortening of this process has obvious benefits. Smeulders et al.(2009) achieved the synthesis of non-functionalized benzene-bridged PMO in about 5 h by employing microwave as heating source in the condensation and extraction steps. This work reports for the first time the microwave-assisted synthesis of sulfonic acid-functionalized benzene-bridged PMO. The samples were characterized by P-XRD, TEM, low temperature N2 adsorption/desorption isotherms, FT-IR and solid-state NMR. The oxidized samples were titrated with NaOH to assess the level of SO3H groups incorporated. XRD patterns taken in different steps of the synthesis showed the presence of the bidimensional hexagonal arrangement of mesopores and the molecular scale periodicity of the walls. 13C CP-MAS NMR confirmed the presence of the mercaptopropyl groups. FTIR measurements showed that the surfactant could be extracted by a single 15 min treatment in a microwave oven. The oxidation of the thiol group was performed under microwave radiation in just 30 minutes. It is found that the level of incorporation of the thiol group in the co-condensation step is strongly dependent on the stirring time. The microwave radiation apparently does not have a strong effect on the meso- and molecular-scale periodicity of the acid-functionalized materials prepared in this work.
9:00 PM - GG6.3
PtNi Nanostructure Optimization for Electrocatalytic Reduction of Oxygen.
Chao Wang 1 , Vojislav Stamenkovic 1 , Nenad Markovic 1
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractStructure architecture at nanoscale is a challenging task to develop advancednanomaterials. Particularly in the catalyst design for chemical-electrical energy conversion, itbecomes critical in order to achieve both the targets of high catalytic activity and long durabilitysimultaneously. While Pt alloys with 3d transition metals (Fe, Co, Ni, etc.) represents promising highlyactive catalysts, it is yet ambiguous how to build up nanoscale objects of such alloy materials that canprotect the light transition metals from leaching out in hostile electrochemical environments. Here wereport a nanostructure of Pt-bimetallic alloy catalyst which is highly active and stable forelectrocatalytic applications. The nanostructure with a Pt-bimetallic core enclosed by a Pt shell of 2~3atomic layers shows the best performance toward the ORR, which can stabilize the content of nonnoblemetal at ~ 25% in the particles and with a specific activity of over 6 times that for state-of-theartPt/carbon catalyst. In addition, a route of nanostructure evolution is proposed to explain thestructure-dependent catalytic performance base on Monte Carlo simulation. The developed approachtoward nanostructure architecture represents a new direction in design and synthesis of advancednanomaterials with the structure-property correlation optimized for the functional applications.
9:00 PM - GG6.30
Non-stoichiometry, Chemical Expansion, and Electrical Conductivity of Pr Doped Ceria.
Sean Bishop 1 , Jae-Jin Kim 1 , Woochul Jung 1 , Todd Stefanik 1 , Yener Kuru 1 , Di Chen 1 , Meng Qu 1 , Karen Stewart 1 , Krystyn Van Vliet 1 , Harry Tuller 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMany oxide ion conducting materials have the ability to absorb or release oxygen above or below stoichiometry with changes in temperature and oxygen partial pressure (pO2), sometimes resulting in deleterious changes in their electrical and mechanical properties. For example, solid oxide fuel cell (SOFC) electrolytes, such as acceptor doped CeO2, when exposed to a large pO2 gradient (air to fuel), become mixed ionic and electronic conductors (MIEC), partially short circuiting the SOFC, as well as expanding (chemical expansion), resulting in stresses that can lead to mechanical failure. It is therefore important to measure these properties and develop a fundamental model to predict ways in which these effects can be mitigated. Pr doped CeO2 (PCO) is a particularly interesting oxygen ion conductor to study because both Pr and Ce can change valence, meaning that as pO2 is decreased from high to low values, PCO transitions through an MIEC region to an ionic region and then back into an MIEC region while undergoing chemical expansion. In this work, experimental values for chemical expansion and electrical conductivity of bulk PCO samples and the modeling of these properties based on oxygen stoichiometry variations as a function of temperature and pO2 is presented. In addition, preliminary electrical and mechanical properties on the nano-scale, using nano-indentation of thin films, will also be presented.
9:00 PM - GG6.31
Impedance and Modulus Spectroscopy of La0.8Sr0.2MnO3 Prepared by Molten Salt Synthesis.
Youn-Woo Hong 1 , Sin-il Gu 1 , Hyo-Soon Shin 1 , Dong-Hun Yeo 1
1 , Korea Institute of Ceramic Engineering & Technoloy, Seoul Korea (the Republic of)
Show AbstractRecent studies have proposed new materials and cell configurations to reduce operating temperatures, which open opportunities of SOFCs in variety of application use. SOFCs have high operating temperature compared with other fuel cells; therefore the cathode electrode of SOFCs is needed high oxidation safety and ionic conductivity. One of the suitable cathode electrodes for SOFCs is La0.8Sr0.2MnO3 (LSM), but its conductivity decreases due to the formation of second phases during electrode formation process, thus the temperature of electrode formation needs to be decreased during the synthesis of fine LSM powders. To prepare LSM of highly homogeneous and fine powder several preparation techniques based on solution chemistry methods, such as co-precipitation and sol-gel process have been used. However these have very complex process and difficult to achieve a uniform composition. In this study, we have prepared LSM powders by molten salt synthesis method, which is easy to control the particle size and has low synthesis temperature, and sintered to characterize the grain and grain boundary properties using impedance and modulus spectroscopy. The synthesis conditions we have conducted were changed by synthesis temperature (500~700°C) and salts (KCl, LiCl, KF and its mixture). LSM powders prepared by each molten salt solution were sintered at various temperatures (800~1300°C) for 2 hours. Using impedance and electric modulus spectroscopy samples were analyzed the effects of second phase, grain size, porosity, and morphology of microstructure according to various temperatures (500~900°C). And then we prepare the Ni/YSZ/LSM system to measure the performance of SOFC unit cell. We will also discuss the relationship between the LSM synthesized by molten salt method in our system and SOFC unit cell performance.
9:00 PM - GG6.32
Proton Conductivity in Pure and Doped Nanocrystalline Ceria.
Giuliano Gregori 1 , Mona Shirpour 1 , Rotraut Merkle 1 , Joachim Maier 1
1 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractIn the last years, there has been a growing interest towards the protonic conductivity of heavily doped nanocrystalline zirconia and ceria at low temperatures. What makes these materials particularly intriguing is the proposed mechanism of conduction, according to which the grain boundaries promotes proton transport and hinders oxygen vacancies at low temperatures.Here, electrochemical impedance spectroscopy (EIS) measurements were performed on microcrystalline as well as nanocrystalline ceria (undoped, 6 at.% gadolinium-doped and 6 at.% gadolinium-decorated) in dry as well as wet atmosphere. Below 200-250°C, all nanocrystalline samples (with density equal to 93% of the theoretical value) exhibit an enhanced total conductivity under wet conditions, which decreases with increasing temperature. Then, after having reached a minimum, the conductivities increase with temperature according to the water-free situation. Such findings, together with direct current measurements indicate that the nanocrystalline samples exhibit protonic conductivity at low temperatures.Remarkably, the total conductivity values below 200°C are very similar for all three nanocrystalline compositions considered here, indicating that the role of the dopant is negligible with respect to the protonic conduction mechanism. In addition, the hydration behavior obtained from thermo-gravimetric analysis (performed in the same wet conditions of the EIS measurements) reveals a strong water uptake below 200°C, exactly in the same temperature range of the increased protonic conductivity. In light of these results, the role of both the grain boundaries and the residual mesosized porosity with regard to the protonic conductivity of nanocrystalline ceria is discussed.
9:00 PM - GG6.33
Low-temperature Selective CO Oxidation in a Realistic H2-rich Stream by Pseudo Core Shell PtCo Nanoparticles Supported on Macro-mesoporous Alumina.
Bit Na Ju 1 , Hye Sun Shin 1 , Ik Jun Jang 1 , Sung June Cho 1
1 Department of Applied Chemical Engineering, Chonnam National University, Gwangju Korea (the Republic of)
Show AbstractHydrogen is an essential element in the forthcoming advancement in the area of sustainable energy. Hydrogen can be efficiently utilized in energy utilization devices, such as fuel cells. Especially, proton exchange membrane fuel cells (PEMFCs) at low temperatures below 373 K require large amount of hydrogen as fuel without CO to ensure their prolonged life. However, the production of hydrogen largely depends on the reformation of various fossil fuels, alcohols, hydrocarbons etc. Thus, reformulated hydrogen rich streams contain large fractions of CO around 1 vol% with a significant amount of water and CO2. Typically, the reformulated hydrogen contains 60-65 % H2, 1-2% CO, 10-15% water and a balance of CO2. The performance of PEMFCs rapidly degrades because of the presence of CO, which poisons thePt-based anode, in turn decreasing the fuel cell performance. Thus, the CO concentration should be controlled at a level that is as low as possible. The partnership for a new generation of vehicles (PNGV) has set a target CO concentration of 10 ppm for the fuel processor. The selective oxidation of CO in a hydrogen rich stream has been investigated using a wide variety of CuO-, gold-, and novel metal-based catalysts. Until now, none of these catalysts exhibited a satisfactory selective catalytic CO oxidation activity in a hydrogen rich stream with a commitment of water and CO2 at a high space velocity for a prolonged time, 100 hr. Therefore, significant advancements in the catalytic performance, activity and selectivity over preferential oxidation of CO are required.The selective CO cleanup reaction belongs to the surface reaction regime, where the entire reaction occurs on the catalyst surface. Therefore, the mass transfer inside the catalyst pellet is important, and its rate must be comparable to the surface reaction rate, thereby making all of the active sites available for the catalytic reaction because the catalytically active sites are located inside the deep pores. The fast heat and mass transfer are essential especially for the reaction, which is accompanied by the release of a large amount of heat.In this study, pseudo core shell PtCo nanoparticles were supported on macro-mesoporous alumina with high permeability for the selective CO oxidation in order to decrease the CO concentration below 10 ppm in the reformulated hydrogen-rich stream containing water and CO2 from 358-368 K at a high space velocity of around 50,000 - 60,000 h-1. The macro-mesoporous nature of the alumina and the unique structure of the pseudo core shell PtCo nanoparticles were responsible for the superior catalytic performance for more than 100 hr.
9:00 PM - GG6.5
Improved Electrochemical Durability of PtRuAu/C Catalyst Synthesized by Radiolytic Process.
Satoru Kageyama 1 , Akio Murakami 1 , Satoshi Seino 1 , Takashi Nakagawa 1 , Takao Yamamoto 1 , Hideo Daimon 2
1 Department of Management of Industry and Technology, Osaka University, Suita, Osaka, Japan, 2 , Hitach Maxell, Ltd., Ibaraki, Osaka, Japan
Show AbstractThe main barrier to the practical use of direct methanol fuel cell (DMFC) is the high cost of Pt-containing catalysts. Improvements in activity, durability and productivity of synthetic process are required. Although the methanol oxidation reaction (MOR) activity has been significantly improved with Pt-Ru bimetallic catalysts[1], the degradation of the catalysts during the operation still remains as a critical issue for the commercialization of DMFC. Recently, it was reported that Au decoration on Pt and PtRu catalysts using underpotential deposition (UPD) is effective for the durability improvement. However, the UPD is complicated method as an industrial process. In this paper, a simple synthetic process for PtRuAu catalysts is presented. We synthesized carbon-supported PtRuAu/C catalysts with the radiolytic method developed in our laboratory[2]. A glass vial containing water, carbon supports (Vulcan XC-72R, Cabot), metal precursors (H2PtCl6, RuCl3 and HAuCl4), and some additives was irradiated with an electron beam (4.8 MeV, 20 kGy) for several seconds. NaH2PO2 for size reduction of metal grain, DL-tartaric acid as chelating agent, 2-propanol as radical scavenger, and NaOH for pH adjustment were added in the synthetic solution as the additives. Radiation-induced radicals reduced the metal precursors, and the PtRuAu particles were deposited on the carbon supports. After the irradiation, the catalysts were washed and dried. The catalysts were characterized using XRD, XAFS, TEM, STEM-EDS and ICP. The PtRuAu /C contained Au of 0.7-7.6 wt.% and the total metals of 8.1 – 15 wt.%. Metal grain sizes were approximately 2 nm. The durability was examined using CV cycling (0.6-1.1 V vs. NHE, scan rate: 5 mV/sec, 50 cycles). The MOR activity was measured using linear sweep voltammetry. The MOR activity of the PtRu/C (without Au) was degraded by 52% with the CV cycling, whereas Pt0.51Ru0.42Au0.07/C showed no degradation within experimental error. It was shown that the radiolytic process is simple and feasible for mass production of the highly durable PtRuAu/C. These findings contribute to the cost reduction of the anode catalysts for DMFC. [1] M. Watanabe and S. Motoo, J. Electroanal.Chem. Interfacial Electrochem., 60, 267 (1975).[2] S. Seino et al., J. Nanopart. Res., 10, 1071 (2008).
9:00 PM - GG6.6
Nanoporous Metal/Ionic Liquid Composite Electrocatalysts for the Oxygen Reduction Reaction.
Joshua Snyder 1 , Jonah Erlebacher 1
1 , Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractWe will discuss a novel electrocatalyst for the cathodic oxygen reduction reaction (ORR) composed of an unsupported, nanoporous NiPt (np-NiPt) structure formed by dealloying Ni from a NiPt alloy. The current state of the art ORR catalyst is composed of Pt nanoparticles physisorbed onto the surface of larger carbon supports (Pt/C). This particular catalyst architecture is susceptible to active surface area loss over time through corrosion of the carbon support among other mechanisms. We will demonstrate that unsupported np-NiPt is not affected by the same active surface area loss mechanisms and has a high intrinsic activity for the ORR, with a mass activity greater than that of Pt/C. The activity is further enhanced by forming a composite catalyst through impregnation of the pores with a hydrophobic, protic ionic liquid which acts to chemically confine oxygen close to the catalytic surface. With this composite catalyst we have recorded mass activities as high as 0.5 mA/μgPt, significantly higher than Pt/C. It should be noted that this composite structure cannot be formed with nanoparticulate catalysts; rather, it is the composite architecture that allows this material to magnify the high intrinsic catalytic activity of Pt-based materials.
9:00 PM - GG6.7
Polymer Blends for Proton Exchange Membranes.
Qian Ma 1 , Meng Zhao 2 , Jingjing Pan 2 , Joel Walker 2 , Tina Lovato 3 , Kelly McNabb 4 , Jason Meyers 3 , Gaber Rupnik 5 , Thomas Smith 2 , Peggy Cebe 1
1 Physics&Astronomy, Tufts Unversity, Medford, Massachusetts, United States, 2 Chemistry, Rochester Institute of Technology, Rochester, New York, United States, 3 National Technical Institute for the Deaf, Rochester Institute of Technology, Rochester, New York, United States, 4 Mechanical Engineering, Rochester Institute of Technology, Rochester, New York, United States, 5 Chemistry, Gallaudet, Washington, District of Columbia, United States
Show AbstractIn the present research, the crystal structure of PVDF in semicrystalline composite films composed of poly(vinylidene fluoride) (PVDF) and poly[4(5)-vinylimidazole/vinylimidazolium trifluoro-methyl-sulfonyl-imide] (PVIm/VIm^+ TFSI^- ) were studied. In these composites, conditions such as choice of solvent, drying conditions, and thermal treatment can affect the crystal phase, crystallite size and degree of crystallinity of PVDF as well as the distribution of the minor component, poly[4(5)-VIm/VIm^+ TFSI^- ]. Such composites may have potential in fuel cells as high-temperature proton-exchange membranes. PVDF imparts mechanical strength to the blend, and because of its high crystal melting point (T_m > 160°C), should improve the high temperature stability of resulting fuel cell membranes. The long range goal is to make a thin, high strength membrane that will exhibit substantial proton conductivity at high temperature and low relative humidity. Thin PVDF/PVIm-PVIm^+ composite films have been fabricated and the nature of the PVDF crystalline polymorph and % crystallinity have been evaluated as a function of the HTFSI content.
9:00 PM - GG6.8
Enhanced Electrocatalytic Activity of Nanoarchitectured Platinum Based Catalysts-Multiwalled Carbon Nanotubes Electrodes for Ethanol Fuel Cell Reaction.
Amel Tabet Aoul 1 , Mohamed Mohamedi 1
1 , EMT-INRS, Varennes, Quebec, Canada
Show AbstractIn the last two decades, the development of alternative power sources has become a crucial issue of research and development. One of the main goals in this quest is the reduction of emission of greenhouse gases like carbon dioxide (CO2). With this in mind, fuel cells appear to be very promising as power supplies for automotive, portable or stationary applications. Ethanol is a hydrogen-rich liquid and it has high energy density (8.0 kWh/kg). Ethanol can be obtained in great quantity from biomass though a fermentation process from renewable resources like from sugar cane, wheat, corn, or even straw. Biogenerated ethanol (or bio-ethanol) is thus attractive since it will not change the natural balance of carbon dioxide in the atmosphere. This is in sharp contrast to the use of fossil fuels. The use of ethanol would also overcome both the storage and infrastructure challenge of hydrogen for fuel cell applications.This work centers on developing advanced free-standing nanoarchitectured layers including the current collector, the catalyst and the catalyst support for direct ethanol fuel cells (DEFC). These nanoarchitectures are made of nanostructured ultrathin films (NSUTFs) Pt or PtxSny electrocatalysts synthesized directly onto multiwalled carbon nanotubes (MWNTs). The NSUTFs are fabricated by pulsed laser deposition (PLD), whereas MWNTs are grown by chemical vapour deposition (CVD) which by their turn are directly grown on the current collector substrate that is composed of highly porous 3D networks of microfibers (~7 micrometer diameter). The formation of NSUTF catalysts were studied as a function of the PLD deposition conditions background atmosphere, i.e. vacuum vs. helium (He) gas, the catalyst composition and its morphology. In this talk, we will report the structure-electrocatalytic properties relationship associated with these nanoarchitectures. These insights were obtained using a myriad of physico-chemical characterization techniques such as SEM, TEM and HR-TEM, XPS, MicroRaman, and XRD combined with electrochemical studies for ethanol electrooxidation, an electrochemical reaction that is central to DEFC technology. We will further demonstrate that such free-standing nanoarchitectures display not only an enhanced electrocatalytic activity but an excellent durability as well.
9:00 PM - GG6.9
Enhanced Electrocatalytic Activity of Oxygen Reduction Reaction on Glancing Angle Deposited Platinum Nanorod Arrays.
Wisam Khudhayer 1 , Ali Shaikh 2 , Tansel Karabacak 3
1 Department of Applied Science, Engineering Science and Systems, University of Arkansas at Little Rock, Little Rock, Arkansas, United States, 2 Chemistry Department, University of Arkansas at Little Rock, Little Rock, Arkansas, United States, 3 Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas, United States
Show AbstractIn this work, the electrocatalytic activity of oxygen reduction reaction (ORR) on vertically aligned, single-layer, low loading, carbon-free, and single crystal Pt nanorods has been evaluated utilizing cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques in a 0.1 M HCLO4 solution at temperatures ranging from 22 to 60 oC. Glancing angle deposition (GLAD) technique was used to fabricate Pt nanorod arrays on glassy carbon (GC) electrode. Electrodes of conventional carbon supported Pt nanoparticles (Pt/C) were also prepared for comparison with Pt nanorods for their electrochemical properties. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were utilized to study the morphology and crystallography of Pt nanorods. SEM and XRD results reveal that Pt nanorods are well-isolated, vertically aligned, and single-crystal with atomically sharp tips. The single-crystal property allows enhanced electrochemical activity and reduced surface oxidation, while the isolated nature of the rods in lateral directions can provide a channeled porosity for effective transportation of gases in a PEM fuel cell. CV results show that Pt nanorod electrocatalyst reduces oxygen to water at a more positive potential that of Pt/C, indicating that our catalyst has a lower oxygen overpotential due to the enhanced electrode porosity, single-crystal property, and the dominance of the preferred crystal orientation for ORR. In addition, a series of CV scans show that our catalyst is more stable than Pt/C in the acidic environment. Finally, in order to get a fair comparison for high surface area catalysts, detailed thin-film rotating disk electrode measurements at temperatures ranging from 22 to 60 oC were performed on Pt nanorods as well as Pt/C for comparison to calculate the most important kinetics parameters (Tafel slopes, electron transfer rate constant, activation energy, Pt mass-specific activity, and area-specific activity), which are the accepted measures of true catalysts activity towards ORR. These results reveal the enhanced mechanism and kinetics of ORR on Pt nanorods compared to Pt/C.
Symposium Organizers
Ting He ConocoPhillips
Karen Swider-Lyons Naval Research Laboratory
Byungwoo Park Seoul National University
Paul A. Kohl Georgia Institute of Technology
Harry L. Tuller Massachusetts Institute of Technology
GG7: Alternative Fuel Cells I
Session Chairs
Wednesday AM, December 01, 2010
Back Bay A (Sheraton)
9:30 AM - GG7.1
Hybrid Acid/Alkaline Electrolyte Fuel Cells.
Murat Unlu 1 , Hyea Kim 1 , Junfeng Zhou 1 , Irene Anestis-Richard 1 , Paul Kohl 1
1 , Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractProton exchange membrane fuel cell (PEMFC) has several desirable features including well-established membranes, electrode assemblies, and cell designs. Although PEMFC has been used in numerous applications, it faces many obstacles including the high cost of platinum catalyst and slow kinetics for the electrochemical reactions. Anion exchange membrane fuel cell (AEMFC) has the potential to address many of the problems facing proton-based cells. The high pH environment in AEMFCs provides faster kinetics for both anode and cathode reactions and paves the way for use of non-noble catalysts, such as silver and nickel. Although AEMFCs have several advantages compared to PEMFCs, the lower ionic conductivity of AEMs is a concern because it may lower the performance. Moreover, the dependence of AEM conductivity on humidity and the need for water in the cathode reaction are significant challenges that limit the performance of current AEM fuel cells.In an effort to use the high conductivity and established infrastructure of PEMs and still exploit the advantages of high pH electrode operation, we have recently developed a hybrid fuel cell comprised of AEM-based electrodes and PEM core. The high pH electrode (AEM electrode) was made using an anion exchange ionomer (AEI) composed of poly (arylene ether sulfone), functionalized with quaternary ammonium groups. The AEM electrodes were pressed onto a PEM membrane, Nafion 212. A critical aspect of this AEM/PEM/AEM cell is the creation of two ionic junctions formed at the interfaces between the proton conducting core membrane and alkaline electrodes. Since these two junctions are in the opposite direction, the junction potentials cancel each other, resulting in a thermodynamic cell voltage of 1.23 V. These junction potentials are accompanied with water dissociation and formation at the boundaries, significantly altering the water dynamics in an operating fuel cell. The performance of the hybrid membrane electrode assembly was evaluated for the H2/O2 system operating at 60oC. At 100% RH, the maximum power density was 55.6 mWcm-2. The power density increased to 78.3 mWcm-2 when the cell was operated with dry gases.The unique water management in the hybrid cell enables self-hydration of the MEA when operated with dry gases because water is formed near the location where it is consumed. To assist in understanding the characteristics of the operating hybrid cell, AC impedance spectroscopy was used to diagnose the impedance of the cell components. The source of the limitations in cell performance will be discussed.
9:45 AM - GG7.2
The Hydrogen-halogen Reversible Fuel Cell for Energy Storage.
Jason Rugolo 1 , Michael Aziz 1
1 , Harvard, Cambridge, Massachusetts, United States
Show AbstractThe hydrogen-halogen reversible fuel cell (HHRFC) is being developed to make affordable grid-scale and intermittent renewable electricity storage. In charge mode, hydrohalic acid is electrolyzed, forming hydrogen and elemental halogen. In discharge mode, these products are recombined to form hydrohalic acid. The advantages of this cell include high round-trip efficiency, inexpensive reactants, and high energy density. This set of chemistries provides many materials challenges, given the corrosive nature of halogens and their aqueous acids, and the high potentials incurred during electrolysis. Electrocatalytic performance of several different candidates for chlorine and bromine reduction catalysis will be presented, including Pt, RuO2, and C. HCl(aq) and HBr(aq) fuel cell configurations will be analyzed, with particular attention paid to the proton exchange membrane (PEM). Round trip efficiencies exceeding 75% in an in-house PEM hydrogen-chlorine fuel cell will be presented and prospects for even better performance will be discussed.
10:00 AM - **GG7.3
Overview of the Status of Carbon Fuel Cells.
Michael Antal 1
1 Hawaii Natural Energy Inst., Univ. of Hawaii, Honolulu, Hawaii, United States
Show AbstractCarbon fuel cells can generate electricity from biocarbon (i.e. biomass charcoal) – as well as from coal, and other fossil carbons – with a theoretical thermodynamic efficiency of 100%. A recent EPRI study indicates that carbon fuel cells have the potential to convert carbons into electrical power at a system level efficiency of about 60%, which is over 20% higher than the efficiencies realized by current state-of-the-art integrated gasification combined cycle (IGCC) or advanced pulverized coal power generation systems. In this presentation I shall review the history and summarize the current status of high-temperature molten-carbonate carbon fuel cells. Afterwards, I will present recent findings in my laboratory concerning the design and performance of a low-temperature aqueous-carbonate/alkaline biocarbon fuel cell. The unusual and poorly-understood properties of biocarbons enable electrochemical-oxidation of carbon in a fuel cell at much lower temperatures than heretofore realized. My presentation will conclude with a summary of our current understanding of the unique chemical and physical properties of biocarbons as related to their performance in carbon fuel cells.
10:30 AM - **GG7.4
State of the Art in Solid Acid Fuel Cells.
Sossina Haile 1 , Mary Louie 1 , Aron Varga 1 , Ayako Ikeda 1 , Calum Chisholm 2
1 , California Institute of Technology, Pasadena, California, United States, 2 , SAFCell, Inc., Pasadena, California, United States
Show AbstractThe compound CsH2PO4 has emerged as a viable electrolyte for intermediate temperature fuel cells. This material is a member of the general class of compounds known as solid acids or acid salts, in which polyanion groups are linked together via hydrogen bonds and monoatomic cations provide overall charge balance. Within this class, several solid acids display a so-called superprotonic transition, at which the compound transforms to a structurally disordered phase of high conductivity. At the transition the conductivity jumps by 3-5 orders of magnitude and the activation energy for proton transport drops to a value of ~ 0.35 eV. The rapid proton transport in the superprotonic phase results from the high degree of polyanion rotational disorder. In the case of CsH2PO4 the transition occurs at 228 °C (with the conductivity rising to 2.2 x 10-2 S/cm at 240 °C), enabling fuel cell operation at temperatures between 230 and 260 °C. The physical characteristics of CsH2PO4 imply a number of realized and potential advantages for fuel cell operation relative to polymer, solid oxide, and liquid electrolyte alternatives, and have begun to push the technology out of the laboratory into commercial development. We present here an overview of the proton transport characteristics of solid acids and the current status of solid acid fuel cell technology.
11:30 AM - **GG7.5
Research Status and Challenges of Alkaline Exchange Membrane Fuel Cells.
Rongrong Chen 1
1 , IUPUI, Indianapolis, Indiana, United States
Show AbstractRecently, there has been a growing interest worldwide in the study of alkaline exchange membrane fuel cells (AEMFCs). AEMFCs inherit some of the key characteristic advantages of both traditional liquid alkaline fuel cells (AFCs) and proton exchange membrane fuel cells (PEMFCs). Still in their early stage of research and development, the AEMFCs still show lower power density than that of the AFCs or the state-of-the-art PEM fuel cells. To fully demonstrate the advantages of the AFCs and PEMFCs in the AEMFCs, several technical challenges need to be addressed. One of the keys to the successful development of AEMFCs is to improve AEMs with high OH- ionic conductivities, good chemical/thermal stabilities and good mechanical integrity. Since 2007, we have started our AEM research and developed our first generation AEMs, poly(ether-imide)- and polysulfone-based AEM systems. AEMFCs based on these AEMs showed comparable or better power performances than what have reported by other research groups using the similar AEMs. Most of the non-noble metals, such as Ag, Co, Ni, or Fe, are not suitable as catalysts in PEM fuel cells because these non-noble metals are not stable in acid media, but are applicable in an alkaline environment. Through electrochemical measurements, we have identified several catalysts, such as carbon supported nano Ag or Ag alloys, Co- and Fe-phthalocyanine, and nano-Pd, that not only have ORR electrocatalytic activities comparable to Pt/C, but also have excellent methanol/ethanol tolerance. The performance of our AEMFCs using MEAs containing the non-Pt cathode catalysts vs. Pt/C has been studied. The ionomers used in the AEMFCs are significantly different in chemical compositions and polymeric structures compared to the Nafion ionomers used in the PEMFCs. We have found that it is very important to develop high performance and stable ionomers for achieving an optimal catalyst-ionomer-gas interface for improved performance of AEMFCs.
12:00 PM - **GG7.6
On the Development of Alkaline Polymer Electrolyte Fuel Cells.
Lin Zhuang 1
1 Department of Chemistry, Wuhan University, Wuhan China
Show AbstractModern electrochemical devices for energy conversion, such as fuel cells, have tended to use solid polymer electrolytes (SPEs) instead of traditional liquid solutions. Nafion has been the most widely used SPE in proton exchange membrane fuel cells (PEMFC). Albeit very successful, Nafion is a strongly acidic polyelectrolyte that only allows noble metals to be used as the catalysts in electrochemical devices. The dependence of noble metal catalysts is not only a matter of price but also an issue of resource limit. To fundamentally get rid of the dependence of noble metal catalysts, alkaline polymer electrolytes (APEs) should, in principle, be used.The development of APEs is quite challenging. First, the chemical stability of quaternary ammonia group is not as good as that of sulfonic group. Second, the mobility of OH- is about one third of that of H+, thus high concentration of OH- is required in the polymer so as to gain high ionic conductivity. However, high ionic concentration is always at the expense of low mechanical strength of the membrane. In Wuhan University, we have been working on developing high-performance APEs since 2001. The latest APEs we developed, quaternary ammonia polysulfone (QAPS), turn out to be suitable for applications of fuel cells and electrolyzers. The ionic conductivity is high than 0.01 S/cm at room temperature in pure water, and the crosslinked membrane has excellent mechanical strength and allows for operations at 90oC.With such an advanced APE, we have been working on developing new electrochemical devices including fuel cells and water electrolyzers. For APE-based fuel cells (APEFC), we used chromium decorated nickel as the anode catalyst for hydrogen oxidation and silver as the cathode catalyst for oxygen reduction. The preliminary performance of such an APEFC with non-Pt catalysts was about 50 mW/cm2 at 60 oC using H2 and O2 as the fuel and oxidant, respectively.Details of this work and recent progress will be presented at the meeting.
12:30 PM - GG7.7
Understanding Oxygen Reduction Reaction Process on Transition Metal Macrocyclic Molecule Catalysts in Alkaline Solution from Density Functional Theory Calculations.
Guofeng Wang 1
1 Department of Mechanical Engineering, Indiana University Purdue University Indianapolis, Indianapolis, Indiana, United States
Show AbstractCatalytic FePc and CoPc molecules have been studied extensively as catalysts for oxygen reduction reactions (ORR), particularly for fuel cell application in alkaline solution. Despite much progress, reaction mechanisms of ORR on the FePc or CoPc catalysts at a molecular level have not been well understood yet. By applying density function theory (DFT) calculations to studying the adsorption process of reactant, intermediate, and product molecules on the FePc or the CoPc surface, we achieved a fundamental understanding of the ORR mechanisms catalyzed by the FePc and CoPc. Specifically, we determined the fully optimized structure and energy for O2, H2O, OH, HOOH, and H2OO molecules adsorbed on the surface of the FePc and CoPc molecules using DFT method. By relating our theoretical results with published experimental observations, we concluded for ORR on FePc and CoPc that (1) O2 adsorption process is the limiting step affecting the kinetics of ORR, (2) OH adsorption process determines the durability of FePc and CoPc, and (3) H2O2 adsorption process distinguishes the four-electron and two-electron routes of ORR. Our study provides new guiding principles on how to tailor the chemistry to attain better macrocycle molecule catalysts for solid electrolyte alkaline fuel cells.
12:45 PM - GG7.8
Development of Efficient Direct Electron Transfer Nanostructured Electrodes for Glucose-air Enzymatic Biofuel Cell.
Mouna Moumene 1 , Dominic Rochefort 2 , Mohamed Mohamedi 1
1 , INRS, EMT, Varennes (Québec), Quebec, Canada, 2 , Université De Montréal, Montréal, Quebec, Canada
Show AbstractBiofuel cells (BFC) are alternative energy devises based on bioelectrocatalysis of natural substrates by enzymes or microorganisms. In terms of applications, BFC will most likely be use in miniature cells to derive power from biological macromolecules to power small devices. It may be envisaged to implant miniature BFCs within a human patient to power micro sensor/transmitter devices e.g., glucose sensors for diabetics, to monitor blood pressure, temperature, metabolite concentrations, or to power a pacemaker or bladder control valve. Enzymatic Biofuel cell (EBFC) utilizes the redox enzymes as a biocatalyst. The redox enzymes, which are separated and purified from an organism, participate in the electron transfer chain that occurs between the substrate and the anode and the cathode by oxidizing the fuel (commonly glucose) and reducing oxygen, respectively. However, redox enzymes are incapable of direct contact with the electrode since their redox centers are insulated from the conductive support by the protein matrices. To contact these enzymes with the electrode, mediators, which are dependent on the class of oxidative enzymes, are utilized. Glucose oxidase (GOx) and Laccase (Lc) are commonly used as an anode and cathode catalysts, respectively in EBFC. Critical challenge in the successful development of a practically valuable GOx and Lc electrodes for biofuel cells is an effective electrical communication between enzyme molecule and electrode surface. The problem associated with many oxidoreductase enzymes is the necessity of using mediators to facilitate the electrons exchange between the enzyme and the electrode. This naturally adds substantial cost, complexity to the system and decreases the current density of the electrode. EBFCs based on direct electron transfer (DET) from the enzyme to the electrode are of great interest to BFC development, because it eliminates the need for mediators making electrode fabrication simpler. This presentation will address some of our efforts into developing efficient DET routes. The approach we develop is based on the electrochemical functionalization of carbon nanostructures substrates. The functionalization procedure will create defects and oxygen functional groups that will insure successful immobilization (anchoring) of the enzyme. In this talk, we will report, the electrocatalytic interactions of glucose and oxygen with the functionalized multiwalled carbon nanotubes-GOx/or Lc built architectures in a pH 7.0~7.3 phosphate buffer/0.1M KCl at room temperature.
GG8: Alternative Fuel Cells II
Session Chairs
Wednesday PM, December 01, 2010
Back Bay A (Sheraton)
2:30 PM - **GG8.1
Improving the Efficiency of Enzymatic Biofuel Cells.
Shelley Minteer 1
1 Chemistry, Saint Louis University, St. Louis, Missouri, United States
Show AbstractEnzymatic biofuel cells have employed a variety of fuels, including: glucose, fructose, methanol, ethanol, glycerol, lactate, pyruvate and ethylene glycol. However, the majority of those fuel cells had low energy density and efficiency. This is primarily due to the use of only a single enzyme to partially oxidize (two electrons) the fuel. This paper will describe the use of multi-enzyme cascades for deep or complete oxidation of biofuels at the anode of enzymatic biofuel cells. Deep or complete oxidation is a relatively new research area for enzymatic biofuel cells, but it is necessary to increase energy density . The paper will also describes new materials science techniques for improving substrate channeling and minimizing product inhibition effects, which are critically important issues with enzyme cascades at electrode surfaces.
3:00 PM - GG8.2
Sodium Hydride as Alternative Energy Having Both Hydrogen Absorption and Hydrogen Generation and Hydrogen Fuel Cycle.
Masataka Murahara 1 , Toshio Ohkawara 2
1 Professor emeritus, Tokai University, Hiratsuka-shi, Kanagawa, Japan, 2 , Mirai Network Co., Ltd., Nerima-ku, Tokyo, Japan
Show AbstractHydrogen is an environmentally friendly fuel, which emits neither CO2 nor radioactivity. Hydrogen itself is light, but the gas cylinder for storage and the absorption alloy are very heavy and unsuitable for transportation. Oil and coal are relatively light and suitable for long-haul transportation and long-term storage.If hydrogen is solidified at room temperature or under atmospheric pressure, its long-distance transportation and long-term storage become possible. It is, then, considered to convert hydrogen into sodium metal. Sodium metal is produced by electrolysing seawater salt or rock salt and stored in kerosene to transport to a consumption place; if water is added to the sodium metal, a large amount of hydrogen can be generated instantaneously anywhere. Besides, a good thing is that the melting point of sodium hydride produced by reacting with hydrogen gas in the process of sodium metal production is 800 degrees C, 8 times higher than 98 degrees C of sodium metal’s, which reduces its handling risk extremely. When adding water, the sodium hydride hydrolyses vigorously to generate hydrogen in the same manner as sodium metal; the amount of the hydrogen generated is twice as large as the hydrogen produced by the reaction of sodium metal and water. That is, sodium hydride is a material that posses functions of both hydrogen absorption and hydrogen generation. Particularly, by-products such as fresh water, hydrochloric acid, sulfuric acid, and magnesium produced in the process of manufacturing sodium metal from seawater, and sodium hydroxide produced at the time of hydrogen generation are the things to have been produced by consuming a large amount of electricity. These by-products are obtained without using power; nothing but these by-products will pay, and the economic effect is huge. The electricity to be consumed for molten salt electrolysis is covered by natural resources for energy production such as offshore wind power, ground wind power, and photovoltaic power generation. Another characteristic of this hydrogen production system is that sodium hydride can be reproduced by molten salt electrolysis of sodium hydroxide, a waste. By repeating the hydrogen generation and the molten salt electrolysis of sodium hydroxide, sodium hydride can be reproduced several times. The supply of raw materials is not necessary at all in the same manner as the nuclear fuel cycle. The most different point of this method from the nuclear fuel cycle is not to produce high-level nuclear waste.As a preparatory experiment, sodium metal was prepared by molten salt electrolysis of sodium hydroxide, and a pressure of one atmosphere of hydrogen was applied while heating the sodium metal at 300 degrees C in a fused silica pipe to produce white yellowish sodium hydride powder. The sodium hydride powder was put in kerosene for storage, and then water was poured to generate hydrogen; it was confirmed that two moles of hydrogen were generated.
3:15 PM - GG8.3
The Performance of M@Pt5Rh (M=Ag, Au) Core-shell Catalysts for Ethanol Electrooxidation.
Zhi Wen Chia 1 , Jim Lee 1
1 Chemical & Biomolecular Engineering, National University of Singapore, Singapore Singapore
Show AbstractAlloy catalysts exhibit different catalytic behaviors from pure metal catalysts due to a variety of reasons, such as electronic effects, ligand effects, ensemble effects, and geometric effects, which often work in unison to give rise to an overall catalytic outcome. In an effort to investigate the effects of electron affinity on the catalyst activity for ethanol electrooxidation, core-shell catalysts supported on Vulcan XC-72 carbon were prepared by a simple step-wise reduction process.In this study, Pt5Rh was used as the shell of the catalyst, with the core comprising of either Au or Ag. The Au@Pt5Rh/C and Ag@Pt5Rh/C catalysts were prepared by first synthesizing the respective Au and Ag cores in water with NaBH4 and using sodium citrate as the protective agent. The cores were then used as the seeds in the subsequent reduction of the Pt5Rh shell by a polyol method. TEM images of the various catalysts showed that metal nanoparticles with typical sizes of 5-6 nm were uniformly distributed on the carbon surface. XRD analysis of Au@Pt5Rh/C and Ag@Pt5Rh/C showed distinct peaks of the respective core metals but diffractions due to Pt and Rh were not detectable. This is expected as the shell was designed to be only 1-2 atomic layers in thickness. Cyclic voltammetry indicated that both the Au-cored and Ag-cored catalysts performed better than catalysts containing only the shell component (i.e. Pt5Rh). There was more performance improvement with the Au-cored catalyst than with the Ag-cored catalyst. The ratio of the forward oxidation peak to the backward oxidation peak, If/Ib, which was taken as an indication of the catalyst’s resistance to poisoning, was doubled for the Au-cored catalyst compared to the alloy-only catalyst and the Ag-cored catalyst.Both Ag and Au strain the lattice of Pt and Rh in the same way, and thus both would raise the d-band centre with respect to the Fermi level of Pt. A raised d-band centre promotes the adsorption of reactants and intermediates, and this results in increased reactions at the catalyst surface. However, this advantage is opposed by the increased difficulty in desorbing the intermediates and products. In the case of the Au-cored catalyst, the presence of a more electronegative core could withdraw electrons from the shell; thereby weakening the adsorptive bond. In the contrary case of the Ag-cored catalyst, electrons are being donated to the shell, which only serve to further promote the bonding between the adsorbates and the catalyst surface. Thus it would appear that there is an optimal balance between the two effects that could result in the enhancement of the catalytic activity for ethanol electrooxidation, as exemplified by the Au-cored catalyst.
3:30 PM - GG8.4
Anodes for Glucose Fuel Cells Made of Carbonized Nanofibers with Embedded Carbon Nanotubes.
Yachin Cohen 1 , Sabina Prilutsky 1 , Pinchas Schechner 3 , Eyal Zussman 2 , Eugenia Bubis 3
1 Chemical Engineering, Technion, Haifa Israel, 3 Electrical and Electronic Eng., ORT-Braude College, Karmiel Israel, 2 Mechanical Engineering, Technion, haifa Israel
Show AbstractElectrodes made of carbonized polyacrylonitryle nanofibers, with and without embedded multiwall carbon nanotubes (MWCNT) were fabricated by the electrospinning (ES) process and evaluated as anodes in a glucose fuel cell (FC). The effect of several processing and structural characteristics, such as the presence of MWCNTs, polymer concentration in the ES solution and silver electroless plating, on FC performance were measured The carbon electrodes were successful as anodes showing significant activity even without additional silver catalyst, with noticeable improvement by incorporation of MWCNTs. The orientation of graphitic layers along the fiber axis and the coherence of layer packing were shown to be important for enhanced electrode activity. The maximal values of open circuit voltage (OCV) and peak of power density (PPD) of unmetallized electrodes, 0.4 V and 30 μW/cm2, were found for composite carbon nanofiber electrode. Electroless silver metallization leads to enhanced performance. Maximal values of OCV and PPD of silvered electrodes were measured to be about 0.9 V and 400 μW/cm2. Thus, carbonized nanofibers with embedded MWCNTs may form a good basis for glucose FC anodes, but better metallization and cell-configuration allowing proper mixing are required.
3:45 PM - GG8.5
The Effect of a Modified Carbon Matrix on the Performance and Durability of Direct Methanol Fuel Cell Catalysts.
Katherine Hurst 1 , Justin Bult 1 , Arrelaine Dameron 1 , Kevin O'Neill 1 , Timothy Olson 1 , Kenneth Neyerlin 1 , Svitlana Pylpenko 2 , Steven Christensen 1 , Ryan O'Hayre 2 , Huyen Dinh 1 , Thomas Gennett 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Colorado School of Mines, Golden, Colorado, United States
Show AbstractDirect methanol fuel cells (DMFCs) are of great interest especially for portable power applications, due to the high energy density of the liquid methanol fuel. However, while an acceptable catalyst, the Pt1-xRux alloy system, has been identified with proven performance via several possible mechanisms, further optimization and improved durability are still essential to attain predicted performance levels. In order to maximize catalyst mass activity and minimize ruthenium crossover, we consider catalyst composition, concentration and particle size as a function of the binding interaction with modified carbon supports. Specifically we have investigated the interaction between the Pt1-xRux alloy catalyst particles with ion implanted and/or directly synthesized boron, nitrogen and fluorine doped carbon substrates. Carbon powder substrates of various surface areas and morphologies, such as carbon black, carbon nanofibers and zeolite-templated carbon via CVD were considered. Through either the implantation or CVD process, the characterization via BET, XPS, XRD and electroanalytical measurements found that the incorporation of the heterogeneity into the carbon matrix alters the physiochemical properties in several ways, including but not limited to: pore size distribution, surface area, surface composition and reactivity. The deposited Pt or Pt1-xRux catalyst on HOPG showed improved durability with a lack of particle migration and coalescence. In the powder systems, the Pt1-xRuxalloy catalyst particles were solution phase deposited from H2PtCl6 and RuCl3 via microwave deposition or incipient wetness (electroless deposition). Within the presentation, the catalyst deposition conditions, such as power, temperature and pressure for microwave process that that led to uniform particle distribution and enhanced catalytic activity will be presented. The full characterization and half-cell performance of these new materials shows that modifying the substrate electronic environment by doping, alters the interaction with the catalyst metal, and in turn the methanol reactivity.
GG9: Solid Oxide Fuel Cells I
Session Chairs
Wednesday PM, December 01, 2010
Back Bay A (Sheraton)
4:30 PM - GG9.1
A Fuel-flexible Anode for Solid Oxide Fuel Cells.
Lei Yang 1 , Shizhong Wang 1 , Yongman Choi 2 , Wentao Qin 1 , Blinn Kevin 1 , Haiyan Chen 3 , Meilin Liu 1
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 , Brookhaven National Laboratory, Upton, New York, United States, 3 , New Jersey Institute of Technology, Newark, New Jersey, United States
Show Abstract Unlike polymer electrolyte fuel cells, solid-oxide fuel cells (SOFCs) can generate electricity from a wide variety of fuels including hydrocarbon fuels, coal gas, and gasified renewable fuels (e.g., bio-fuels). However, the conventional anode for an SOFC, a composite consisting of nickel and yttria-stabilized-zirconia (YSZ), is highly susceptible to carbon buildup (coking) and deactivation (sulfur poisoning) by contaminants commonly encountered in readily available fuels [1]. We report a mixed ion conductor, BaZr0.1Ce0.7Y0.2-xYbxO3-d, that allows rapid transport of both protons and oxide ion vacancies [2]. It exhibits high ionic conductivity (1.3 to 6.4x10-2 Ω-1cm-1) at relatively low temperatures (500 to 700 oC). Its unique ability to resist deactivation by sulfur and coking appears linked to the mixed conductor’s enhanced catalytic activity for sulfur oxidation and hydrocarbon cracking/reforming, as well as enhanced multilayer water adsorption capability. Furthermore, synchrotron-based X-ray analyses and electron microscopy reveal that nano-sized BaO islands grow on the Ni surface. The nanostructured BaO/Ni interfaces readily adsorb water and facilitate water-mediated carbon removal reactions. Density functional theory (DFT) calculations predict that the dissociated OH from H2O on BaO reacts with C on Ni near the BaO/Ni interface to produce CO and H species, which are then electrochemically oxidized at the triple-phase boundaries of the anode [3]. Reference:[1] A Atkinson et al, Nature Materials 3, 2004[2] Lei Yang et al, Science 326, 2009[3] Lei Yang et al, Unpublished
4:45 PM - GG9.2
Catalytic Activity Enhancement for Oxygen Reduction on (La0.5Sr0.5)2CoO4-decorated Surfaces of Epitaxial (La1-xSrx)CoO3 Films for Solid Oxide Fuel Cells.
Ethan Crumlin 1 , Eva Mutoro 1 , Sung-Jin Ahn 1 , Gerardo Jose la O' 1 , Michael Biegalski 2 , Hans Christen 2 , Yang Shao-Horn 1
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 CNMS, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractRecent research efforts have focused on developing SOFCs for auxiliary and distributed power applications around 500°C, where SOFC cost, shortened start-up time and SOFC degradation can be reduced. The main barrier to achieve acceptable SOFC efficiency at intermediate temperatures is the voltage loss associated with oxygen reduction reaction (ORR) at the cathode. Lack of fundamental understanding in the ORR mechanism at the molecular level limits the design of new cathode materials with higher activity. We have recently found enhanced ORR activity on (001)-oriented La0.8Sr0.2CoO3 (LSC113 80-20) surfaces relative to bulk in films that are epitaxially grown on 9.5 mol% Y2O3-stabilized ZrO2 (YSZ) (001) single-crystals. In this work, we investigate how catalytic enhancement can be obtained by varying the surface coverage of (La0.5Sr0.5)2CoO4 (LSC214) on top of epitaxially grown LSC113 80-20 and La0.6Sr0.4CoO3 (LSC113 60-40) films. LSC113 80-20 and LSC113 60-40 films with varying amounts LSC214 surface coverage in the range from ~1/4 mono-layer coverage to ~27 mono-layers were deposited on 20 mol% Gd2O3 doped CeO2 /YSZ (001) using pulsed-laser deposition (PLD). The ORR activity in terms of surface oxygen exchange rate was investigated using electrochemical impedance spectroscopy (EIS) over a range of oxygen partial pressures from 10-4 atm to 1 atm at 550°C. We show that the surface exchange rates on LSC113 films and LSC214-decorated LSC113 surfaces are enhanced with respect to bulk LSC113 as well as explore potential mechanisms that may be responsible for the increase in activity.
5:00 PM - GG9.3
Enhancement for Oxygen Reduction Reaction on Epitaxial (La,Sr)CoO3-δ Films for Solid Oxide Fuel Cells.
SungJin Ahn 1 , Ethan Crumlin 1 , DongKyu Lee 1 , Eva Mutoro 1 , Michael Biegalski 2 , Hans Christen 2 , Yang Shao-Horn 1
1 , MIT, Cambridge, Massachusetts, United States, 2 , Oak Ridge National Lab., Oak Ridge, Tennessee, United States
Show AbstractReduction of operating temperatures is of vital importance to shorten start-up time and reduce degradation of components in solid oxide fuel cells (SOFCs). Decreasing the operating temperature, however, reduces the SOFC conversion efficiency since the kinetics of electrochemical reactions, especially the oxygen reduction reaction (ORR), are thermally activated. Lack of fundamental understanding in the ORR mechanism at the molecular level limits the design of new catalysts with highly enhanced activity and conversion efficiency. In this study, we examine ORR activity (surface oxygen exchange kinetics) on (001)-oriented La0.8Sr0.2CoO3-δ (LSC 80-20) and La0.6Sr0.4CoO3-δ (LSC 60-40) films. Both of LSC 80-20 and LSC 60-40 films were epitaxially grown on 9.5 mol% Y2O3-stabilized ZrO2 (YSZ) (001) single-crystals, with an epitaxial buffer layer of 20 mol% Gd-doped CeO2 (GDC) using pulsed-laser deposition. The thicknesses of these films are varied from 20 nm to 150 nm. Crystallinity, epitaxial relationships, and strains of LSC/GDC/YSZ (001) films were analyzed using 4-circle X-ray diffraction. ORR activity of the films was investigated using electrochemical impedance spectroscopy at 520oC under varying oxygen partial pressures between 10-4 atm to 1 atm. In contrast to the epitaxial LSC 60-40 films showing similar catalytic activity with bulk, the surfaces of epitaxial LSC 80-20 films exhibit markedly increased ORR activity up to two orders of magnitude in comparison to bulk, which may be attributed to increased oxygen vacancy concentrations in the film. The mechanism of ORR activity enhancement will be discussed.
5:15 PM - GG9.4
Model Thin Film SrTi1-xFexO3-δ Mixed Conducting Cathodes for Solid Oxide Fuel Cells:Correlation between Performance and Materials Properties.
WooChul Jung 1 , Veronika Metlenko 2 , Roger De Souza 2 , Harry Tuller 1
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Institut für Physikalische Chemie, RWTH Aachen University, Aachen Germany
Show AbstractProgress in achieving improved solid oxide fuel cell (SOFC) performance, particularly at reduced temperatures (<600C), has been constrained by our imperfect understanding of the kinetics controlling the cathode processes and the resultant inability to further reduce cathode polarization loss. In this work, a new perovskite materials system is selected, offering the ability to systematically control both the levels of ionic and electronic conductivity as well as the band structure. This, in combination with considerably simplified electrode geometry, is demonstrated to provide improved insight into the SOFC cathode processes. For this purpose, dense thin film SrTi1-xFexO3-δ cathodes, with controlled levels of x, prepared by PLD, were investigated primarily by impedance spectroscopy (IS) and additionally by oxygen isotope exchange experiments with subsequent Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis, as a function of electrode geometry, temperature, pO2, and deposition condition. The STF exhibited low area specific resistance (ASR) with surface exchange coefficient values, k, comparable in magnitude to those exhibited by other popular mixed conductors. Surprisingly the magnitude of k was found to be only weakly dependent on the magnitudes of the electronic and ionic conductivities (over nearly five orders of magnitude change in σel). The observed trends are discussed in relation to the defect, transport, electronic band structure, and surface chemical properties of the STF thin film electrodes, concluding with suggestions regarding the criteria required for good MIEC SOFC cathode performance.
5:30 PM - **GG9.5
Tailoring Grain Boundaries of Yttrium-doped Barium Zirconate Electrolytes for Intermediate Temperature Solid Oxide Fuel Cells.
Enrico Traversa 1
1 MANA, National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractHigh cost, together with the long-term stability due to the high working temperatures, are the main hurdles for the widespread use of solid oxide fuel cells (SOFCs). Reducing the SOFC operating temperature in the 400-700°C range can decrease fabrication costs and overall improve performance. This aim can be obtained using high temperature proton conductor (HTPC) oxides as electrolytes, due to by their lower activation energy for proton conduction (0.3-0.6 eV), with respect to oxygen-ion conductor electrolytes. Moreover, proton conductor electrolytes offer the advantage of generating water at the cathode, and thus the fuel does not become diluted during cell operation.Among HTPCs, doped BaCeO3 electrolytes show the largest proton conductivity, but they are not suitable for fuel cell application since they easily react with acidic gases, e.g. CO2, and water vapor. Differently, doped BaZrO3 offer excellent chemical stability, but low conductivity values are obtained for sintered pellets. The lower electrical conductivity is the consequence of poor conductive grain boundary regions, coupled with the difficult sintering of the material. This work reports the various strategies recently followed in our lab to improve the conductivity of Y-doped barium zirconate, towards the limitation of grain boundary surface, the use of co-doping for improving the sinterability and grain boundary conductivity, and the fabrication of films by pulsed laser deposition (PLD).
GG10: Poster Session: Solid Oxide Fuel Cells, Alternative Fuel Cells and Diagnostic Techniques
Session Chairs
Thursday AM, December 02, 2010
Exhibition Hall D (Hynes)
9:00 PM - GG10.1
Prediction of Solid Oxide Fuel Cell Cathode Activity from First-principles Descriptors.
Yueh-Lin Lee 1 , Dane Morgan 1 2 , Jesper Kleis 3 , Jan Rossmeisl 3
1 Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark, Kgs. Lyngby Denmark
Show AbstractCathode activation polarization is a major source of overpotential and lost efficiency in solid oxide fuel cells (SOFC), a problem that will likely become worse as next generation SOFCs are pushed to lower operating temperatures. Perovskites are the major class of materials used for modern SOFC cathodes due to their ability to catalyze the oxygen reduction reaction (ORR). However, difficulties in characterizing perovskite oxide surface structures under operating conditions impede the development of robust structure-property relationships and materials design rules for optimizing the ORR on perovskites. One computational strategy for optimizing ORR is to identify easy to calculate descriptors that can predict the catalytic activity. Such an approach has been very valuable in understanding, predicting, and optimizing catalytic activity in low-temperature ORR catalysts. In this talk we use ab initio methods to show that a bulk electronic structure/energetic property is capable of describing the experimental ORR activity of perovskite SOFC cathodes. This result opens the door to first-principles design of new SOFC cathode materials.
9:00 PM - GG10.10
Anionic-cationic Bi-cell Design for Direct Methanol Fuel Cell Stack.
Hyea Kim 1 , Murat Unlu 1 , Junfeng Zhou 1 , Irene Anestis-Richard 1 , Paul Kohl 1
1 , Georgia Institute of Technology, Atlanta, Georgia, United States
Show Abstract To achieve higher voltage than values obtained from a single fuel cell, and high power-density, multiple fuel cells can be connected in series in a stack. Chu et al. published a bi-cell stack design. It could reduce the overall system volume due to the use of a common fuel tank. However, in the case of the DMFC application, there is a concern about electrochemical short circuit caused by the potential difference between anode 1 and cathode 2. The liquid methanol fuel provides an ionic path for anode 1 to act as the anode to cathode. This results in a self-discharge mechanism and loss of fuel efficiency. This same short circuit can also occur in the monopolar stack. In order to address this concern, a new fuel cell stack design is demonstrated using an anion exchange membrane (AEM) fuel cell and a proton exchange membrane (PEM) fuel cell in series using a common liquid fuel. In the alkaline fuel cell, the potentials are shifted to more negative values as a result of the high pH. The electrode potentials for both acid-PEM and alkaline-AEM were evaluated. It is shown that the actual AEM cathode 2 are essentially at the same potential as the PEM anode 1 which avoids an undesired potential difference and resulting loss in current between the two electrodes. Further, the complimentary direction of water transport in the two kinds of fuel cells simplifies water management at both the anodes and cathodes. The effect of ionomer content and ion exchange capacity on the AEM electrode potential and the activity of methanol oxidation were investigated using half-cell tests. The open circuit potential of the anionic-cationic bi-cell was 1.36 V with 2.0 M methanol fuel and air at room temperature.
9:00 PM - GG10.11
A Low Temperature Solid Oxide Fuel Cell Based on Zr /Sm Co-Doped Nanocomposite Electrolyte.
Rizwan Raza 1 , Bin Zhu 1
1 Energy Technology, Royal Institute of Technology, KTH, Stockholm Sweden
Show AbstractA nanocomposite electrolyte Zr/Sm co-doped ceria coated with KCO3/Na2CO3 was synthesized by a co-precipitation method. The electrochemical study of the two phase nanocomposite electrolytes with carbonate coated on the doped ceria shows the high oxide ion mobility at low temperature (300-600oC) solid oxide fuel cell (LTSOFC). The interface between two constituent phase studied by electrochemical impedance spectroscopy (EIS). Ionic conductivities were measured with EIS. The morphology and structure of composite electrolyte were characterized using field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The fuel cell power density was 700mW/cm2 and an open-circuit voltage of 1.009 V, are achieved at low temperature (400-580 °C). This co-doped approach with 2nd phase gives the good idea to achieve the challenges of solid oxide fuel cell (SOFC).
9:00 PM - GG10.12
Optimization of Cathode Material (Ba0.5S 0.5)1-x AxBO3 for Intermediate Temperature SOFC using High Throughput Experimentation (HTE).
Sooyeon Seo 1 , Heejung Park 1 , Chan Kawk 1 , Kee-Sun Sohn 2
1 Materials Research center, SAIT, Samsung Electronics, Giheung, Yongin 446-712 Korea (the Republic of), 2 Printed Electronics Engineering, Sunchon national university, Sunchon Korea (the Republic of)
Show AbstractMixed ionic-electronic conducting perovskite type oxides of (Ba 0.5Sr 0.5)1-x AxBO3 ( A = Li, Sm, Nd ; B = Co, Fe, Mn, Ti, Zn, Ni ) were studied by solid state high-throughput experiment (HTE) that was reinforced by a computational optimization process. They are key materials that find use in intermediate temperature(500 - 700'C) solid oxide fuel cell cathode because these have high mobility of the oxygen vacancies and strong ionic conductivity. Electrolyte-supported symmetric (Ba 0.5Sr 0.5)1-x AxBO3 /GDC/ScSZ/ GDC/ (Ba 0.5Sr 0.5)1-x AxBO3 cells consisting of (Ba 0.5Sr 0.5)1-x AxBO3 cathodes, a GDC buffer layer, and a ScSZ electrolyte were fabricated, and the electrochemical performance of the cathode was investigated at intermediate temperature using AC impedance spectroscopy. Acquiring of a good cathode material that exhibits both desirable ionic conductivity and compatible thermal expansion coefficients with adjacent layer in cells is challenging. Systematic combinatorial synthesis and optimization strategy based on computational algorithms greatly facilitate rapid identification of promising engineered cathode materials.
9:00 PM - GG10.13
Power Generation from Biomass-derived Ethylene Glycol and Glycerol over PtRuSn/C Electrocatalysts.
Hyung Ju Kim 1 , Sung Mook Choi 1 , Sara Green 2 , Geoffrey Tompsett 2 , Seon Hwa Lee 1 , George Huber 2 , Won Bae Kim 2
1 Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 2 Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts, United States
Show AbstractRecently, biomass-derived oxygenated hydrocarbons, such as ethanol, propanol and ethylene glycol have been demonstrated as potential fuels in the DMFC-like fuel cells, since such biomass-derived molecules are renewable and CO2 neutral. Ethylene glycol and glycerol, which can be produced from biomass, would be promising because they are less toxic and inflammable, and also possess relatively high theoretical energy density. In particular, glycerol is a good potential hydrogen source for fuel cells given a rapid growth of biodiesel production. More importantly, glycerol can be produced in a renewable, environmental-friendly, and cost-effective manner.In this study, we investigate the structural, electronic and electrochemical features of carbon-supported ternary PtRuSn (PtRuSn/C) catalysts for the electrooxidation of ethylene glycol and glycerol. The ternary PtRuSn/C catalyst is characterized by various physicochemical analyses such as X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and X-ray absorption-near-edge spectroscopy (XANES). The ternary PtRuSn catalyst shows noticeable modifications on the catalyst phases from Pt or PtRu in regards of the structural and electronic features, such as a change in lattice parameter and electronic modification in unfilled d band states of Pt atoms. Such a structurally- and electronically-modified PtRuSn/C catalyst substantially enhances the electrocatalytic activities for ethylene glycol and glycerol oxidations, resulting in larger peak currents and lower onset potentials of the electro-oxidations. Structural and electronic modifications, such as changes in lattice parameter and partial filling of the Pt d-band vacancies, affect the activity of the ternary catalyst by facilitating the C–C bond scission and tolerance of CO or CO-like residues. Also, the synergistic effect of Ru and Sn on PtRuSn catalyst could help complete oxidation of CO or CO-like poisonous species on Pt sites, allowing the Pt sites free for ethylene glycol and glycerol oxidation, thus leading to significant improvements of electrocatalytic activity and stability.
9:00 PM - GG10.15
Direct Observation of Degradation Induced Structural and Chemical Changes in “Pt3Co” Fuel Cell Catalyst Nanoparticles by Cs-Corrected Scanning Transmission Electron Microscopy.
Christopher Carlton 1 , Lawrence Allard 2 , Yang Shao-Horn 1
1 Department Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractPt alloy nanoparticles are used as catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs). However, catalyst degradation during PEMFC operation, which leads to both loss of electrochemical area and decreased specific activity, has significantly limited PEMFC lifetime. It has been proposed that the reduced specific activity of “Pt3Co” has been attributed to a weaker electronic modification effect caused by increasing Pt shell thickness on “Pt3Co” nanoparticles. So far, direct observation of thickening of Pt-enrichment surface regions has been lacking, mainly due to the difficulty of obtaining sub-nanometer scale chemical resolution. To this end, energy dispersive spectroscopy (EDS) is used in an aberration-corrected scanning transmission electron microscope (STEM) to image the spatial distribution of Pt and Co in PtCo transition metal alloy nanoparticle catalysts. The elemental distributions in pristine catalysts are compared to the elemental distribution of extensively cycled catalysts. Direct evidence of shell thickening and other catalyst degradation mechanism will be reported and discussed. This research at the Oak Ridge National Laboratory's High Temperature Materials Laboratory was sponsored by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program.
9:00 PM - GG10.16
Model-based Optimal Experimental Design of Fuel Cells Patterned Electrodes.
Francesco Ciucci 1 , Thomas Carraro 1 , William Chueh 2 , Wei Lai 3
1 Applied Mathematics, University of Heidelberg, Heidelberg Germany, 2 Materials Science, Caltech, Pasadena, California, United States, 3 Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan, United States
Show AbstractImpedance spectroscopy (IS) is a tool utilized in many areas of electrochemistry for the study of physical-chemical phenomena occurring in devices ranging from fuel cells to batteries. An improvement of data quality gathered in IS can be achieved by coupling the experimental procedure with physically-based models. To this end, it is key to study the influence of the physical parameters that describe the model. In addition, the model has to be refined by a fitting procedure with experimental data. Here we study both aspects using tools from optimal experimental design (OED).In this work we consider the study of a ceria patterned anodes, both in the thin film [1] and in the slab configuration [2]. Typically the parameter estimation is performed by least squares fitting the impedance data. In the experiment, a sample with a given geometry is tested by performing a frequency sweep (from microhertz to megahertz).In this work, OED techniques are used to optimize the experiment by reducing the error covariance matrix of the parameter set. The key idea is to define design and experiment parameters, in this case, dimensions of the patterned electrode and the frequencies. Subsequently the experiments are performed under those conditions that reduce the covariance matrix (a functional of it), hereby increasing the information content of the considered parameters. On one hand we attempt to reduce the covariance of the parameters by tweaking the geometry and by optimally distributing the testing frequencies. On the other hand we consider the concurrent minimization of the experimental time and of the covariances of the parameters. The result of this optimization procedure is the definition of experimental setup that can improve the quality of the estimated parameters and at the same time drastically reduce (up to 2 orders of magnitude) the experimental time.[1] W. C. Chueh, W. Lai and S. M. Haile, “Electrochemical Behavior of Ceria with Selected Metal Electrodes,” Solid State Ionics, 179, 1036-1041 (2008). DOI: 10.1016/j.ssi.2007.12.087.[2] W. Lai and S. M. Haile, "Impedance Spectroscopy as a Tool for Chemical and Electrochemical Analysis of Mixed Conductors: A Case Study of Ceria," J. Amer. Cer. Soc. 88, 2979-2997 (2005).
9:00 PM - GG10.17
Improvement of Oxygen Permeation Flux by Coating High Oxide Ion Conductor with Mixed Electronic and Ionic Conductor Thin Film.
Susumu Imashuku 1 , Yang Shao-Horn 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractOxygen transport membranes (OTMs) are ceramic membranes that employ mixed electronic and ionic conducting (MIEC) materials having perovskite (ABO3) or double perovskite (A2BO4) structures. The oxygen transport flux in the membrane is controlled by 1) adsorption and incorporation of oxygen on the surface, also known as surface oxygen exchange, and 2) the transport of oxygen in the ceramic lattice. When the membrane is below a critical thickness Lc, no further reduction in thickness will increase oxygen flux due to surface oxygen exchange limitation. Lc is defined as the ratio of D/k, where D is the oxygen ion diffusivity (cm2 sec-1) in the bulk and k is the surface oxygen exchange coefficient (cm sec-1). Our approach to enhancing OTM performance is to fabricate supported thin-film membranes of MIECs of ABO3 and A2BO4 having thicknesses below Lc to effectively maximize oxygen flux through the OTM and lower oxygen separation operating temperature. In this study, we examined La0.6Sr0.4Co0.2Fe30.8O3-δ (LSCF6428) and La2NiO4+δ (LNO) as a thin-film membrane supported on 8% yttria stabilized zirconia (8YSZ). YSZ is a high oxide ion conductor, but their oxygen permeation fluxs are very low (less than 1.0 × 10-11 mol cm-2 sec-1 at 700°C) due to its low electronic conductivity. Interestingly, the oxygen permeation flux of 8YSZ pellet of 100 mm, whose all surfaces were coated with LSCF6428 (1.0 × 10-8 mol cm-2 sec-1 at 700°C), was much higher than that of 8YSZ pellet alone, where an activation energy (0.62 eV) comparable to that of 8YSZ for ion conduction (0.79 eV) was found. It is proposed that the enhanced oxygen permeation can be attributed to the fact that oxide ions go into 8YSZ and electrons go into LSCF6428, where oxide ion conduction in 8YSZ is a limiting step for oxygen permeation. This hypothesis is further supported by the fact that 8YSZ pellet whose all surfaces were coated with silver was found to have much higher oxygen permeation flux ( 3.0 × 10-9 mol cm-2 sec-1) than 8YSZ pellet whose top and bottom faces only were coated with silver (1.0 × 10-11 mol cm-2 sec-1) at 700°C. The mechanism for the enhancement of the oxygen permeation flux of 8YSZ pellets with surface decoration will be discussed in detail.
9:00 PM - GG10.18
Microstructure and Chemical Evolution of Ni-based SOFC Anodes after Operation at 800oC in Synthesis Gas Fuel Containing 10 ppm PH3.
Yun Chen 1 , Harry Finklea 1 , Song Chen 2 , Xueyan Song 2 , Chunchuan Xu 3 , John Zondlo 3 , James Poston 4 , Kirk Gerdes 4
1 Department of Chemistry, West Virginia University, Morgantown, West Virginia, United States, 2 Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia, United States, 3 Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia, United States, 4 National Energy Technology Laboratory, US Department of Energy, Morgantown, West Virginia, United States
Show AbstractIn the anode-supported solid oxide fuel cell (SOFCs) system, Ni-YSZ cermet is commonly employed as the anode-active and support material. Ni-YSZ is known to convert synthesis gas (syngas) fuel to electricity at high efficiency at elevated temperatures. However, during the operation, some trace contaminants in coal syngas fuel such as PH3 could have a significant impact on Ni-YSZ anode performance and even result in the degradation of cell power over a few hundred hours.In this study, three commercial cells with a Ni/YSZ composite anode were operated at 800C in a syngas mixture containing 10 ppm PH3. Three conditions including 20 h operation without electrochemical load (Cell-1), 20 h operation at a constant current of 0.5 A/cm^2 (Cell-2), and 117 h operation at 0.5 A/cm^2 (Cell-3), were applied. At the beginning and the end of each operation, electrochemical impedance spectra (EIS) were collected. Cell-1, which was exposed to PH3 at open circuit, exhibited the largest increase in the polarization resistance. Evidently, current flow is not necessary for degradation.After operation, the cells were examined using XPS, SEM and TEM associated with EDS and EELS. The interfaces and boundaries of the anode, including the interface between Ni-P products and the anode, the interface between the YSZ electrolyte and Ni grains, and the boundaries between Ni, YSZ, and pores in the anode active layer were carefully imaged. Preliminary inspection of SEM and TEM images show no obvious changes in grain structure and morphology of the active layer as a function of exposure time and current flow. XPS did not detect phosphorus in the active layers of the 3 cells. TEM work indicates that, within short exposure period (Cell-1), phosphorus species were not detected in the anode active layer and electrolyte/anode interface. On Cell-3, at the anode/fuel side, a ~10 micron thick discontinuous Ni-P dense layer was formed during the exposure. The layer was composed of columnar Ni-P grains close to the original anode surface and large grains on the outer surface. Underneath this Ni-P layer, there was another porous 10 micron thick layer composed of a YSZ grain network. Ni was mostly depleted in the layer. The change in grain structure indicates that the growth of columnar grain was inhibited possibly due to the limited amount of reactants, especially of nickel. Based on the above microscopy work and the EIS results, the mechanism of PH3 attack on the Ni/YSZ anode in SOFC cells during the operation will be discussed.
9:00 PM - GG10.19
Evolution of Microstructure and Chemistry of Solid Oxide Fuel Cells upon Operation.
Xueyan Song 1 , Yun Chen 2 , Song Chen 1 , Harry Finklea 2 , Gregory Hackett 3 , Kirk Gerdes 3
1 Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia, United States, 2 Department of Chemistry, West Virginia University, Morgantown, West Virginia, United States, 3 National Energy Technology Laboratory, US Department of Energy, Morgantown, West Virginia, United States
Show AbstractYttria-stabilized zirconium (YSZ) based solid oxide fuel cells (SOFCs) are typically composed of a YSZ electrolyte, a (La,Sr)MnO3 (LSM) and YSZ composite as the cathode and a porous composite of nickel-YSZ cermet as the anode. Electrochemical reactions take place on the triple-phase boundary (TPB) between the YSZ electrolyte, the electrode and the fuel gas phase. Both intrinsic and extrinsic defects such as dopants and vacancies, presumably randomly distributed within the YSZ lattice, are the dominant factors for the ionic conductivity of YSZ. Small changes in TPB chemistry can drastically modify the SOFC oxygen exchange rate and affect SOFC performance and lifetime. During operation of SOFCs at elevated temperature, certain impurities and dopants segregate to grain boundaries, and the chemistry and atomic structure of electrolyte/electrode interface also changes. These changes affect the subsequent electrochemical reactions.In this present work, systematic microstructure and chemistry analysis on SOFC TPB were performed using Transmission Electron Microscopy (TEM). The TEM imaging is carried out on cross-sectional samples, with the focus on analyzing the YSZ from electrolyte and composite electrodes, and the interfaces of YSZ/Ni and YSZ/LSM in the active layer of anode and cathode, respectively. Primary results highlighted in this paper include the following points: (1) YSZ within the electrolyte and anode composite has a different atomic structure than YSZ in the cathode. Instead of a random distribution of dopants and oxygen, distinct long-range ordering is observed in the YSZ from the anode and electrolyte. Such long range ordering persists in the cells operated under electrical load at 800oC for 550 h. (2) In the anode side active layer and TPB area, a layer of ribbon NiO grain covers the entire interface between Ni and YSZ. By contrast, this interfacial NiO ribbon phase was not found in the grain boundaries between neighboring Ni grains. The thickness of the NiO ribbon phase increases from about 10 nm in cells operated for 24 h to over 60 nm in cells operated for 550 hours. (3) For the active layer on the cathode side, in cells running on pure hydrogen for 24 h and 550 h respectively, the Mn/La ratio increased significantly in YSZ grains, across the range of about 60-90 nm away from the YSZ/Ni interface. Since the grain size of YSZ and LSM are about 300 nm, such a large amount of Mn migration from LSM into the YSZ will likely impact the ionic conductivity of YSZ.
9:00 PM - GG10.2
Material Selection and System Design for Liquid Metal Anode Solid Oxide Fuel Cells (LMA-SOFCs).
Harry Abernathy 1 , Kirk Gerdes 1
1 Energy System Dynamics, National Energy Technology Laboratory, Morgantown, West Virginia, United States
Show AbstractReplacement of a conventional ceramic solid oxide fuel cell (SOFC) anode with a liquid metal anode (LMA) permits direct conversion of solid or liquid fuel (e.g., coal, biomass, JP-8) to electricity. LMA-SOFC systems feature simplified fuel processing units with reduced fuel conditioning costs, but suffer from diminished power density relative to a traditional SOFC. The LMA performance can be improved by enhancing oxygen transport through the liquid metal layer or by decreasing the interfacial resistances between the anode and fuel. The impact of LMA material selection and system design on these two performance parameters will be described. Current research at the National Energy Technology Laboratory is directed at experimental and theoretical determination of the kinetic parameters of a liquid metal anode to evaluate the ultimate capabilities and limitations of an LMA-SOFC system.
9:00 PM - GG10.21
In situ X-ray Studies of La0.6Sr0.4Fe0.8Co0.2O3 Epitaxial Thin Films as Model Solid Oxide Fuel Cell Cathodes.
Kee-Chul Chang 1 , Brian Ingram 2 , Hui Du 3 , Paul Salvador 3 , Daniel Hennessy 1 , Hoydoo You 1
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States, 3 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractWe characterized epitaxial thin-films of La0.6Sr0.4Fe0.8Co0.2O3 (LSCF) grown on single crystal yttria-stabilized zirconia (YSZ) substrates with and without a Gd doped ceria (GdC) buffer layer. The system was designed as a model cathode of the solid oxide fuel cell (SOFC), which has advantages of high efficiency and fuel-flexibility but is not yet economically competitive enough to gain widespread acceptance. We used a combination of X-ray reflectivity, diffraction, florescence and spectroscopy to fully characterize the cathode between 500-800°C with applied potential, simulating SOFC operating conditions. Possible correlations between the cation oxidation states and the thin film structure with annealing time and applied potential will be presented.
9:00 PM - GG10.22
Ab-initio Study of Defect Thermodynamics in Zirconium Substituted Ceria.
Chirranjeevi Balaji Gopal 1 , William Chueh 1 , Axel van de Walle 1 , Sossina Haile 1
1 Materials Science, California Institute of Techology, Pasadena, California, United States
Show AbstractCeria and its doped oxides find important applications in automotive catalysts and as electrolytes in fuel cells It has been experimentally demonstrated that the oxygen vacancy formation and migration energy in ceria is reduced by the addition of zirconium, enhancing its oxygen storage capacity. Apart from enthalpy reduction, entropic effects caused by local vacancy ordering have also been shown to affect the redox properties. We use density functional theory with Hubbard term corrections (DFT+U) to study the thermodynamic properties of Zr substituted ceria as a function of composition. Using supercell calculations, the effect of Zr substitution on the vacancy formation energy is investigated. The interactions among Ce^3+, Ce^4+, Zr and vacancies - treating them as individual species - is calculated and used to construct a so-called cluster expansion effective Hamiltonian. This Hamiltonian can be used to efficiently model the thermodynamics of defect complexes and local vacancy ordering, thus providing insights into defect interactions in ceria-zirconia solid solutions.
9:00 PM - GG10.23
In Situ X-ray Studies of La0.6Sr0.4Co0.2Fe0.8O3-δ at Elevated Temperatures under Controlled Oxygen Partial Pressure and Electrochemical Loading.
Paul Fuoss 1 , Timothy Fister 1 2 , Brian Ingram 2 , Dillon Fong 1 , Matthew Highland 1 , Peter Baldo 1 , Jeffrey Eastman 1 , Hui Du 3 , Paul Salvadore 3
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Chemical Sciences and Engineering, Argonne National Laboratory, Argonne, Illinois, United States, 3 Dept. of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractThe performance of solid oxide fuel cells (SOFCs) is strongly influenced by the nanoscale structure and chemistry of electrode materials under operating conditions. However, because SOFCs are operated at elevated temperatures and at near-atmospheric pressure, care is required in the utilization and interpretation of traditional surface science studies, which typically involve vacuum conditions near room temperature. Using in situ x-ray scattering and spectroscopy technologies to bridge this experimental gap, we have measured equilibrium structures of SOFC cathode materials at elevated temperatures and controlled oxygen partial pressures, and examined the dynamic structural changes that occur at the cathode side of a fuel cell under conditions that simulate actual operating conditions. We have recently studied the bulk and surface structures of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) grown as strained, single crystal thin films (approximately 20 nm thick) on SrTiO3 (STO), NdGaO3 (NGO), DyScO3 (DSO), Y2O3-stabilized ZrO2 (YSZ), and gadolinium doped ceria (GDC) on YSZ. We have found strong segregation of Sr to the surface of LSCF (001) films and the presence of strain-dependent reconstructions of the LSCF (001) surface. This talk will focus on our most recent results that measure the time evolution of the lattice parameter and conductivity of LSCF films grown on GDC-coated YSZ (001) substrates in response to cathodic overpotential. We find systematic increases in the LSCF lattice parameter with decreasing pO2 and with cathodic overpotential. The conductivity of the film falls rapidly during application of a cathodic bias and exhibits abrupt, temperature-dependent changes suggestive of ordering transitions.
9:00 PM - GG10.24
In-situ Observation of Skin-layer Formation in Pt3Ni Nanoparticles.
Miaofang Chi 1 , Chao Wang 2 , Nenad Markovic 2 , Karren More 1 , Vojislav Stamenkovic 2
1 Material Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractAlloying Pt with 3d transition metals (Fe, Co, Ni, etc.) are strong candidate catalysts for the oxygen reduction reaction (ORR) in fuel cells.1-3 Previous work on well-defined extended surfaces has demonstrated that ultrahigh ORR activity can be achieved for bi-metallic alloys with Pt segregated surfaces, e.g., a Pt-skin layer and a Ni enriched subsurface layer in the case of Pt3Ni (111).4 After that a lot of efforts in the literature are trying to achieve the same structure architecture at nanoscale, where however many fundamental questions remain unanswered. For example, how is the skin layer formed, how does the nanoparticle size, shape, and composition affect its formation, and what is the preferred temperature or/and temperature range to best control the formation of the skin layer at the atomic level? Such nanoparticle studies are challenging, primarily due to the requirement of both spatial resolution (for Å-scale imaging and analysis) and catalyst particle stability during in-situ annealing. In this presentation, recent work on the observations of the structural and compositional evolution of Pt3Ni nanoparticles (~5nm) during in-situ heating in an aberration-corrected Scanning Transmission Electron Microscope (STEM) will be discussed. Atomic-scale STEM imaging is simultaneously combined with Electron Energy Loss Spectroscopy (EELS) in order to reveal the atom re-arrangement during heating. These experiments were performed using a probe-corrected FEI Titan 60/300 TEM/STEM equipped with a Gatan Quantum 965 spectrometer, and a Protochips, Inc. AduraTM heating holder.5 Our results in combination with electrochemical studies provide persuasive information about the nanostructure-property correlation in electrocatalysts, which also has great potential toward guiding the further development of other advanced functional nanomaterials.References:1. Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M., J. Electrochem. Soc. 1999, 146 (10), 3750-3756.2. Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M., J. Am. Chem. Soc. 2006, 128 (27), 8813-8819.3. Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M., Nature Materials 2007, 6 (3), 241-247.4. Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M., Science 2007, 315 (5811), 493-497.5. The microscopy research performed at the ORNL SHaRE User Facility supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.
9:00 PM - GG10.25
Characterization of Thin-film Ceria-zirconia for Applications in Solid Oxide Fuel Cell Anodes.
Yong Hao 1 , William Chueh 1 , Chirranjeevi Gopal 1 , Linda Nazar 2 , Sossina Haile 1
1 Materials Science, California Institute of Technology, Pasadena, California, United States, 2 Chemistry, University of Waterloo, Waterloo, Ontario, Canada
Show AbstractOxygen vacancy formation in CeO2 and doped ceria has been extensively studied owing to the wide use of ceria, spanning from materials for solid oxide fuel cell anodes to solar-thermochemical energy storage. Substituting Zr into CeO2 to form Ce1-xZrxO2-δ solid solutions has been found to facilitate the formation of oxygen vacancies in the bulk and improve oxygen ion mobility, which are critical properties for these applications. However the key factors leading to these enhancements are not well understood. Surface reaction kinetics and defect thermodynamics of Ce1-xZrxO2-δ (x < 0.5) thin films (< 100 nm) were probed by impedance spectroscopy. Thin-film symmetric cells with embedded Pt pattern current collectors were fabricated by photolithography and pulsed laser deposition, and their electrochemical properties were examined in reducing atmospheres between 550 and 650°C. The surface activity is sensitive to the presence of silica on the surface as probed by XPS and angle-resolved XPS; and its segregation energetics and/or kinetics appear to depend on the Zr substitution level. Overall, a decrease in the surface resistance is exhibited with increasing Zr concentration. The capacitance of the thin films gives a direct measure of the oxygen nonstoichiometry, with negligible contributions from interfacial effects. Similar to bulk materials, the oxygen nonstoichiometry in thin-film ceria-zirconia is significantly higher than undoped and rare-earth doped ceria under comparable temperatures and oxygen activities. The nonstoichiometry obtained through impedance spectroscopy in this work exhibits excellent agreement with literature data obtained through other approaches. The dependence of the capacitance on oxygen partial pressure varies significantly from that exhibited by CeO2 or samarium doped ceria, suggestive of complex mechanisms for oxygen vacancy compensation.
9:00 PM - GG10.26
Methane-fueled Micro Solid-oxide Fuel Cells operating below 600°C.
Bo-Kuai Lai 1 , Kian Kerman 1 , Shriram Ramanathan 1
1 , Harvard School of Engineering and Applied Science, Cambridge, Massachusetts, United States
Show AbstractMicro-SOFCs (μSOFCs) have been receiving increasing research attention because of their enormous potential as long lasting, rapid re-charging, lightweight portable power sources. They have been demonstrated to exhibit good power density at temperatures as low as 400 oC. To date, μSOFCs have been demonstrated with pure hydrogen. However, for μSOFCs to be cost competitive and relevant for widespread use it is crucial to explore fuels alternative to H2. In this work, we will report our recent results on methane-fueled μSOFCs. The choice of methane (CH4) is due to it being a constituent of natural gas, which is relatively widely available. Different materials (Pt, Pd, La0.6Sr0.4Co0.8Fe0.2O3, and La0.3Sr0.7TiO3) in nanostructured form will be studied as the anode; while Yttria-stabilized Zirconia (YSZ) and Pt constitute the electrolyte and cathode, respectively. Fabrication consideration and approaches for improved current-collection for oxide-anode μSOFCs will be discussed. Microstructure of these anode materials under reducing environment and in-situ, high-temperature deformation of the μSOFC membranes and relation to stress evolution will also be presented.
9:00 PM - GG10.28
Proton Conducting SOFC Utilizing Internally Steam Reformed Alcohol Fuels.
Maria Azimova 1 , Steven McIntosh 1
1 Chemical Engineering, University of Virginia, Charlottesville, Virginia, United States
Show AbstractProton conducting oxides provide a promising new approach to lowering the solid oxide fuel cell (SOFC) operating temperature from >973K to the intermediate temperature (IT) range (673–873K). Fuel cells operating in this range would maintain the benefits of high temperature operation, such as increased cell efficiency, fuel flexibility, use of transition metal catalysts, CO and S tolerance, while gaining the benefits of lower temperature operation - increased lifetime, reduced balance of plant material costs and decreased startup/shutdown time. BaCe1-x-zZrx(Y,Yb)zO3-δ perovskites are among the leading materials for application in IT proton conducting SOFC (H+ -SOFC). These materials have shown technologically relevant proton conductivity in the target temperature range. The primary barriers to application until now have been stability in CO2 containing atmospheres, low grain boundary conductivity, and the high sintering temperature (>1973K) required to produce dense electrolytes. Recent research efforts have shown that transition metal doping can reduce the sintering temperature of these materials to <1698K, while maintaining the ionic conductivity of almost an order of magnitude greater than that of one of the best oxygen ion conducting materials, yttria-stabilized zirconia, at 873K. In this study, anode-supported H+-SOFC were fabricated using a dual-layer tape-casting technique and cell performance was demonstrated utilizing both humidified H 2 and methanol/water fuels. All materials were synthesized using a modified Pechini procedure and confirmed as phase pure cubic perovskites by XRD. DC conductivity of 2.1x10-3 S/cm was measured in 3% humidified H2 at 873K. Proton conduction was confirmed by Nernst potential measurements, conducted in a dual chamber system as a function of pH2 and pO2 driving force and the materials were shown to be phase stable in the presence of H2 and CO2 . Fuel cells consisted of a 0.04mm thick electrolyte supported on a 0.3mm thick porous Ni/BCZY anode with an La0.8Sr0.2CoO3-δ /BCZY cathode. The maximum open circuit potential (OCP) of 1.04V was achieved at 873K in 3% humidified H2 fuel with an accompanying peak power density of 89mW/cm2 . The total cell polarization resistance was 0.83Ω.cm2 . Impedance measurements showed decreasing polarization and ohmic resistance with increasing current density.Cell operation via internal steam reforming of methanol was demonstrated using a methanol/water feed with 3:1 and 2:1 S:C ratios; however, the OCP was reduced to 0.97 and 0.95V and peak power density decreased to 65 and 58mW/cm2 respectively. This was primarily attributed to a lower H2 concentration in the feed stream. Methanol reforming rates were also assessed over nickel and copper catalysts and compared for BCZY and alumina supports for varying methanol/water S:C ratios. 1 Azimova, M. A. & McIntosh, S. Transport properties and stability of cobalt doped proton conducting oxides. SSI 180, 160 (2009).
9:00 PM - GG10.4
Oxygen Reduction Reaction in Solid Oxide Fuel Cells: Comparison of Perovskite Cathodes.
Rambabu Sambangi 1 , Xueyan Du 2 , Edwin Walker 3 , Zhiping Luo 4 , Jingbo Liu 1
1 Chemistry, Texas A&M University-Kingsville, Kingsville, Texas, United States, 2 College of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou, Gansu, China, 3 Chemistry, Southern University and A&M College, Baton Rouge, Louisiana, United States, 4 The Microscopy and Image Center, Texas A&M University, College State, Texas, United States
Show AbstractSolid oxide fuel cells (SOFC) are drawing significant attention due to their advantages of high energy conversion efficiency, flexible fuel supplies and zero/near-zero emission of air pollutants. A critical challenge hindering SOFC technology is the requirement of the high operating temperatures. Therefore, there has been much attention on the development of cathode and electrolyte materials with long life-spans and high conductivities, allowing operation at intermediate temperature. This study presents the performance and nanostructural comparison on two SOFCs half cell devices; one is operating at high temperatures and another one at intermediate ones. The former is composed of lanthanum strontium manganese (SrxLa1-xMnO3, LSM) oxide as cathode and yttrium stabilized zirconia (YSZ) as electrolyte; whereas the latter composed of lanthanum strontium cobalt iron oxide (SrxLa1-xCoyFe1-yO3, LSCF) as cathode and samarium doped ceria (SmxCe1-xO2) as electrolyte. The homogeneous and ultrafine cathodic nanoparticles with high specific surface area were prepared via a colloidal method followed by solid state chemistry. The SOFCs were then constructed by integrating nanoparticles into macro-devices. Nanocharacterization was implemented using transmission/scanning electron microscopy, X-ray powder diffraction and wavelength dispersive spectroscopy. Analytical results depict that the diameter of cathode materials is in the range of 20-50 nm, which results in larger surface area and allows the rapid gas diffusion and instantaneous chemisorption of O2. The highly crystalline and mono-dispersive perovsikte LSCF and LSM nanoparticles with spherical shapes were formed. Elemental composition analysis shows that all elements distribute essentially uniform.The kinetics of SOFC devices were evaluated using oxygen reduction reaction (ORR) rate (io). In this work, the focus has been on establishing a reliable method of determining the mechanism and kinetics of the ORR at triple phase boundary (TPB). Techniques used include three-electrode cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) over a temperature range from 400 to 1000 °C with interval of 50 °C. The applied potential varied from -0.7 to +0.7 V to identify io. EIS was performed to establish resistance-free i/η relationship, when the frequency varies from 0.05-106 Hz. In generalization, the SOFCs composed of LSCF (operating at 400-700 °C) display superior performance (one magnitude higher) that LSM based cathode (600-1000 °C). It was also found that the micro-distortion and vacancy-induced stress of LSM are both greater than those for LSCF, which suggests that the LSCF display faster ionic conductivity and higher structural stability. This presentation will provide in-depth discoveries on the relationship between nanostructure and electrochemical performance.
9:00 PM - GG10.6
Room Temperature Indirect Formic Acid Fuel Cells Enabled by a Dehydrogenation PtRuBiOx Catalyst.
Kwong-Yu Chan 1 , Siu Wa Ting 1 , Huanqiao Li 1 , Fujun Li 1 , Jenkin Tsui 1
1 , University of Hong Kong, Pokfulam Road Hong Kong
Show AbstractA room temperature indirect formic acid fuel cell is enabled by a platinum-ruthenium-bismuth mixed metal/metal oxide supported catalyst developed recently[1]. Dehydrogenation of formic acid occurs at room temperature and atmospheric pressure. The reaction proceeded in aqueous phase is highly selective to hydrogen and carbon dioxide without detection of carbon monoxide. The gas evolution rate is 400 mL per hour at 80°C when 80 mL solution of 15% aqueous formic acid was added to 400 mg catalyst containing PtRuBiOx supported on carbon. The catalysts were prepared by the citric acid gel method using metal precursors of RuCl3.xH2O, H2PtCl6.H2O, and Bi(NO3)3.5H2O dissolved in water and with citric acid added as a protecting agent. Vulcan carbon 72 XC was added to absorb the metal precursors. The water was evaporated and the powder was heated at 850°C in inert atmosphere to obtain the final product catalyst. The catalyst was characterized by Philips Tecnai G2 TEM and XPS. An indirect formic acid fuel cell is built with a fixed bed reforming chamber in which the catalyst is placed. Hydrogen is evolved upon contact of catalysts acid with aqueous formic acid at room temperature. While reforming can proceed at room temperature, hydrogen evolution increases rapidly with temperature as the dehydrogenation reaction of formic acid on the PtRuBiOx catalyst proceeds has a low activation energy barrier of 37.3 kJ/mol. References[1]. S.W. Ting, S. Cheng, K.Y. Tsang, N. van der Laak, and K.Y. Chan Chem. Commun., 2009, 7333–7335
9:00 PM - GG10.7
Understanding Cathode Surface Kinetics of Low Temperature SOFCs with Electrochemical Impedance Spectroscopy and Electrical Conductivity Relaxation Measurements.
Lincoln Miara 1 , Jacob Davis 1 , Tiffany Kaspar 2 , Lax Saraf 2 , Soumendra Basu 1 , Uday Pal 1 , Srikanth Gopalan 1
1 Materials Science, Boston University, Brookline, Massachusetts, United States, 2 Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractAs the operating temperatures of solid oxide fuel cells (SOFCs) are lowered, the cathode kinetics become increasingly torpid. Understanding the reaction kinetics is exceedingly complex, and accordingly, there are still no agreed upon mechanisms. Recently, many studies have analyzed electrochemical impedance spectroscopy (EIS) results on patterned and micro-patterned electrodes. EIS is an invaluable tool for identifying limiting elementary steps in the overall oxygen reduction reaction. Regardless of the ionic conductivity of these materials, these results tend to suggest that, particularly at low operating temperatures, surface processes are responsible for the largest polarization losses. However, elucidating the individual reaction steps responsible for the polarization is not possible with EIS alone. The electrical conductivity relaxation (ECR) experimental technique is a convenient way to measure the surface exchange coefficient (kQ). This parameter is a measure of the catalytic ability of the cathode towards the reduction of oxygen. In this light it is possible to compute the surface exchange coefficient from the surface related area specific polarization resistance (Rc) measured from impedance spectroscopy with the equation kQ =(kT)/(4e2Rcc) where c denotes the oxide ion concentration and k, T and e are the Boltzmann constant, temperature and elementary charge respectively. Thus, whereas Rc includes contributions from both surface exchange properties (i.e. surface adsorption and dissociation) and surface diffusion of atomic oxygen to the TPB, kQ only measures the surface properties. Finally, if samples are sufficiently thin such that the critical length (lc defined as kQ/DQ) is much less than the thickness of the film, the relaxation kinetics determined by kQ only and can be fitted with a simple exponential function. Therefore, ECR experiments are conducted on very thin samples (25nm) of heteroepitaxial La0.85Ca0.15MnO3 (LCM) grown on LaAlO3 (001 LAO) single crystal substrates over the temperature range from 500 - 800°C and in the pO2 range from 10-3 -1.0 atm. These results are compared to the extracted kQ from EIS measurements on patterned cathodes of LCM on YSZ to shed light on the true nature of surface limiting reactions occurring over the measured temperature and partial pressure range. This work allows a deeper understanding of the nature of limiting surface reactions which is critical for developing high power, low operating temperature next generation SOFCs.
9:00 PM - GG10.8
Enzyme Based Biological Fuel Cell for Energy Storage and Generation.
Blaine Butler 1 , James Garrett 1 , Mike Danilich 1
1 , Luna Innovations, Charlottesville, Virginia, United States
Show AbstractThis research presentation focuses on the development of an enzymatic fuel cell utilizing enzymes as catalysts for oxidation and reduction reactions at the anode and cathode respectively. A novel method for enzyme stabilization and coupling to the electrode surface is employed, resulting in increased enzymatic stability during both storage and operation. In order to augment direct electron transfer from the active site of the enzyme to the electrode surface, various electron mediators and conductive fillers have been utilized. Increased electron transfer from the enzyme has expanded the variety of enzymes that can be utilized in these biological fuel cells. Experimental evidence indicates that this technique in combination with appropriate matrix modifiers result in retention of enzyme activity for over 2 months, for a variety of oxidation/reduction enzymes, a significant increase when compared to soluble enzyme under similar conditions. The ability of this technique to create a stable platform for multi-enzyme functionality is also demonstrated, with no negative effect on activity observed, and in some multi-enzyme systems increased activity is observed.
9:00 PM - GG10.9
Novel Alloy Oxide Electrocatalysts for Hydrogen-halogen Fuel Cell.
Sujit Mondal 1 , Jason Rugolo 1 , Michael Aziz 1
1 SEAS, Harvard, Cambridge, Massachusetts, United States
Show AbstractHalogen electrocatalysts are important for halogen-based fuel cells such as the hydrogen-halogen regenerative fuel cell for grid-scale electrical energy storage, as well as for chlorine generation in the chlor-alkali industry. We report a family of nano-structured ruthenium alloy oxides that are good electron conductors, are stable and potent catalysts for halogen reduction and halide oxidation, but have very low precious metal content compared to the commercial Dimensionally Stabilized Anodes (DSAs) used in the chlor-alkali industry. The ruthenium metal fraction in these alloy oxides has been reduced to values as low as 1 atomic percent Ru, with electrocatalytic performance remaining very close to that of conventional DSA materials. Synthesis and the results of electrocatalytic performance and stability tests will be reported.
Symposium Organizers
Ting He ConocoPhillips
Karen Swider-Lyons Naval Research Laboratory
Byungwoo Park Seoul National University
Paul A. Kohl Georgia Institute of Technology
Harry L. Tuller Massachusetts Institute of Technology
GG11: Solid Oxide Fuel Cells II
Session Chairs
Ting He
Karen Swider-Lyons
Thursday AM, December 02, 2010
Back Bay A (Sheraton)
9:30 AM - GG11.1
Grid-supported Large Area Micro Solid Oxide Fuel Cells with Scalable Power.
Masaru Tsuchiya 1 2 , Shriram Ramanathan 2
1 , SiEnergy Systems LLC, Boston, Massachusetts, United States, 2 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractMicro solid oxide fuel cells (μ-SOFC) with nanometer scale electrolytes have attracted significant attention in recent years. Reducing the electrolyte thickness results in a decrease of ohmic resistance, thereby the use of nanoscale thin film electrolytes enables to lower the operation temperature of SOFC to < 600 oC. There have been reports on promising power density of μ-SOFC in the recent few years, however, a rather challenging aspect is scaling the power generating membrane active area. The high power density has been achieved only in free standing membranes with small area (typically 0.01 mm2) due to a large stress originated from the thin film membrane electrode assembly. In addition, the area utilization has been limited (typically < 5%) due to a constraint from KOH etching of Si substrate. In order to make this technology commercially viable, it is necessary to develop novel methods to significantly scale up the total power output and area utilization. In this presentation, we report on the use of nanostructured metal grids to significantly increase an active area of μ-SOFC. Nanometer scale (50-150 nm) 8 mol% yttria doped zirconia (electrolyte) and La0.6Sr0.4Co0.8Fe0.2 O3-δ(cathode) films were deposited on a silicon nitride coated Si (100) wafer at 550 oC by rf sputtering. Metal grids (Pt/Ag) were created by the combination of photolithography and dc-sputtering on the cathode surface. The metal grids work as a mechanical support for nanoscale thin film structure as well as a cathode current collector. The role of grid shape, size and width on the stability of thin film structure will be discussed in the presentation. Following the grid formation on cathode surface, the backside of Si wafers were patterned by photolithography and reactive ion etching, and then etched in 30wt% KOH at 96 oC for 3 hours. Finally, a porous metal layer (Pt/Ni) was sputtered on the backside to form anode. Fuel cell performance was evaluated in a custom design fuel cell test station with 5% H2 mixed with Ar. This approach was successful to fabricate large area μ-SOFC devices with high power output. We were able to form μ-SOFC with an active area larger than 10 cm2 with an area utilization over 80%. The grid supported μ-SOFC with ~ 13 mm2 active area was tested, and open circuit voltage of 0.98 V with power output of ~ 13 mW (corresponding to ~ 100 mW/cm2) was achieved at 570oC.
9:45 AM - GG11.2
New Insights into Reaction-diffusion Pathways in Ceria-based SOFC Anodes.
William Chueh 1 , Yong Hao 1 , Francesco Ciucci 2 , Sossina Haile 1
1 Materials Science, California Institute of Technology, Pasadena, California, United States, 2 Institute of Applied Mathematics & HGS MathComp, Universität Heidelberg, Heidelberg Germany
Show AbstractInvestigating reaction mechanisms in porous fuel cell electrodes is complicated by the random structure and coupling between surface reaction and diffusion. Using pulsed-laser thin-film deposition and metal pattern liftoff lithography, we fabricated two-dimensional M-Sm0.2Ce0.8O1.9 (M = Ni, Pt) composite electrodes with well-defined phase boundaries, carrier diffusion lengths, and reaction site densities. Electrochemical cells were characterized by AC impedance spectroscopy and DC polarization measurements, and the data analyzed via a physically-derived two-dimensional transport model. The combination of periodic microstructures and detailed transport model permits the direct extraction of surface reaction rates, carrier concentration, and diffusivity over a wide range of geometry and environmental conditions. At reaction site densities typical of a solid oxide fuel cell anode, it was found that hydrogen electro-oxidation occurs predominately over the ceria | gas two-phase boundary rather than the ceria | gas | metal triple-phase boundary, with an open-circuit area-specific resistance of 2 Ω cm2 (650 °C, pH2 = 10-1 atm, pH2O = 5 × 10-3 atm, where the area reflects the true surface area). Furthermore, under the idealized thin-film geometry, our results show that reactions take place over the ceria surface at distances well over 100 microns from metal features. We also present electrochemical data on high surface-area ceria-based columnar electrodes that deliver exceptional electrochemical performance in the absence of catalytic metal reaction sites at intermediate temperatures.
10:00 AM - **GG11.3
Thin Film Electrodes on Oxide Electrolytes: Model Systems for Mechanistic Studies and Candidates for Micro-SOFC Cathodes.
Juergen Fleig 1
1 , Vienna University of Technology, Vienna Austria
Show AbstractThin film electrodes on solid oxide ion conductors are highly useful tools when aiming at a better understanding of (electro-)chemical processes taking place in solid oxide fuel cells. Compared to porous electrodes applied in large scale SOFCs, micro-patterned or dense thin film electrodes are less complex in terms of structure and chemistry, enable direct analytical access to the reaction sites, allow simple determination and variation of three phase boundary (TPB) length and electrode or electrolyte surface areas, etc. In this study, these advantages are employed in order to improve the mechanistic understanding of the oxygen reduction kinetics on Pt, (La,Sr)(Co,Fe)O3 (LSCF) and (La,Sr)MnO3 (LSM) electrodes on YSZ electrolytes. In particular, combination of secondary ion mass spectrometry (SIMS) and electrical polarization yielded valuable results: On Pt thin film microelectrodes the width of the electrochemically active TPB was quantitatively determined by cathodic incorporation of 18O into YSZ and analyzing its trace by SIMS. Relatively low temperatures turned out to be crucial in such experiments in order to minimize bulk diffusion of the tracer which can strongly distort the tracer distribution. Finite element calculations allowed estimates on the possibilities and limits of measurements performed to quantify TPB widths on solid ion conductors. On perovskite-type electrodes voltage driven 18O tracer incorporation and SIMS analysis were used to map and visualize resistive zones during oxygen reduction (visible as steps or sharp gradients in 18O concentration). Moreover SIMS and other analytic tools were employed to improve the understanding of structure-property relations of perovskite-type electrodes. Electrochemically highly active and less active electrodes show correlations between surface composition and polarization. Based on this knowledge, we can identify thin film electrodes for which the cathodic polarization resistance is sufficiently low to possibly employ them in future micro-SOFCs, particularly when aiming at fabrication procedures which are compatible with standard micro-technology. Pulsed laser deposition as well as a sol-gel route was used to prepare thin film electrodes exhibiting polarization resistances below 0.2 Ohmcm2 at 600 °C. However, stability issues are still to be solved.
10:30 AM - **GG11.4
Low Temperature Solid Oxide Fuel Cells: A Transformational Energy Conversion Technology.
Eric Wachsman 1
1 Energy Research Center, University of Maryland, College Park, Maryland, United States
Show AbstractThere has been a tremendous effort to lower the operating temperature of solid oxide fuel cells (SOFC) to ≤800°C, for cost and reliability considerations; while simultaneously there has been an even larger effort to increase the operating temperature of proton exchange membrane fuel cells (PEMFC) to >100°C, for performance and fuel poisoning considerations. Somewhere in between is the optimum operating temperature for a fuel cell depending on fuel choice and degree of external fuel processing (vs. relying exclusively on internal reforming). To date there has been limited/negligible success at obtaining a PEM electrolyte capable of operating in this temperature range. In contrast, alternate higher conductivity, and thus lower temperature, oxide electrolytes have existed for some time (ranging from gallates and ceria to bismuth oxide based materials). However, each of these materials has it's own specific issues. Moreover, transitioning from a zirconia electrolyte to one of these alternate electrolytes requires development of an entirely new cell material set.Over the last 20 years we have focused on using the most conductive, and hence lowest temperature, bismuth oxide based electrolyte. To overcome its inherent instability in reducing environments we developed a bilayer bismuth-oxide/ceria electrolyte. To overcome reactivity with more conventional cathode materials we developed a bismuth-ruthenate/bismuth-oxide composite cathode. Finally, these materials were integrated into an anode supported cell resulting in power densities of ~2 W/cm2 at 650°C. These SOFCs have sufficient power down to 400°C, thus attaining the temperature "sweet spot" necessary to transform fuel cell technology. Moreover, since SOFCs can operate on conventional fuels – thereby obviating the requirement for a hydrogen infrastructure that has relegated fuel cells to a "future technology" - this technology has the potential to revolutionize not only stationary power, but also transportation and portable power applications.
11:30 AM - **GG11.5
Thin Film Solid Oxide Fuel Cell Power Skins for Autonomous Systems.
Shriram Ramanathan 1
1 , Harvard, Cambridge, Massachusetts, United States
Show AbstractI will discuss on-going efforts in our laboratory to design, fabricate and test low temperature solid oxide fuel cells (SOFCs) for miniature autonomous systems such as micro-robots. Specifically we will consider the following: materials synthesis for high performance electrodes to enable low temperature catalytic activity, large area ultra-thin oxide membranes for scalable power generation and novel in-situ diagnostics to probe fuel cell membrane morphology evolution during operation. Fascinating results on the correlation between stresses in the membranes during and fabrication and post-release and their topology will be presented. In-situ measurement techniques as well as methods to understand near-surface defect phenomena in thin film oxide components for the solid oxide fuel cells will be presented. Finally, we will consider how interfacial phenomena that can be influenced by space charge or boundary effects may be exploited for low temperature SOFCs.
12:00 PM - GG11.6
Pulse Reactor Studies to Assess Potential of LSCM-based Materials as Direct Hydrocarbon Solid Oxide Fuel Cell Anodes.
Michael van den Bossche 1 , Steven McIntosh 1
1 Chemical Engineering, University of Virginia, Charlottesville, Virginia, United States
Show AbstractSolid oxide fuel cells (SOFCs) are a promising technology for efficient and direct generation of electricity from hydrocarbon fuels. Currently, the challenge for SOFCs is the development of the anode, as the state-of-the-art nickel/yttria-stabilized zirconia (Ni-YSZ) anode performs well in H2, but lacks long-term stability in hydrocarbon fuel due to Ni sintering and graphite formation. Anode prerequisites are many: activity for hydrocarbon oxidation, high oxygen ion and electron conductivity, resistance towards sulfur poisoning, and mechanical and chemical stability at temperatures > 700°C and partial oxygen pressure (pO2) < 10-20 atm.It is a considerable challenge to measure hydrocarbon reaction rates under realistic anode conditions. A typical oxidation rate measurement involves flowing a mixture of fuel and oxygen over a powder catalyst. This is distinctly different from the SOFC anode where no gas-phase oxygen is present – all of the oxygen is supplied via ion diffusion through the anode material. Furthermore, the surface reaction rates must be determined independently of the rate of this bulk diffusion.We have developed a pulse reactor technique to measure hydrocarbon reaction rates under SOFC anode conditions, and applied this method to La0.75Sr0.25Cr1-xMnxO3-δ (LSCM)1 as a potential anode candidate. Oxidation of CH4 was found to occur through a modified Mars-van Krevelen mechanism, with the rate and selectivity depending on Mn concentration and oxygen stoichiometry of the material2. Impedance measurements performed on working fuel cells with LSCM (x = 0.5) anodes showed that electrochemical performance increased with increasing oxygen ion transport through the anode3, confirming that the reaction rate depends on oxygen content.SOFC tests using dense LSCM (x = 0.5) anodes showed that performance in hydrocarbon fuel is catalytically limited4. To increase CH4 reaction rates, Mn was substituted with 10 at% Co, Fe, Ni and V 5 and CH4 reaction rates were determined using the pulse reactor. Oxidation of CH4 was enhanced for LSCMCo and LSCMNi, compared to LSCM, at temperatures ≥ 700°C. Whereas CO2 production on LSCM continuously decreased with oxygen stoichiometry of the catalyst, on LSCMCo and LSCMNi, CO2 was produced in two separate ranges of oxygen stoichiometry, with the additional production peak attributed to the reduction of Co(II) and Ni(II) to their respective metallic phases. (1) Tao, S. W.; Irvine, J. T. S., Nat. Mater. 2003, 2, (5), 320-323.(2) van den Bossche, M.; McIntosh, S., J. Catal. 2008, 255, (2), 313-323.(3) Bruce, M. K.; van den Bossche, M.; McIntosh, S., J. Electrochem. Soc. 2008, 155, (11), B1202-B1209.(4) van den Bossche, M; et al., J. Electrochem. Soc. 2010, 157, (3), B392-B399.(5) van den Bossche, M.; McIntosh, S., Chem. Mater., Submitted.
12:15 PM - GG11.7
Chemical Durability of SOFCs: Equilibrium Consideration and Kinetic Aspects.
Kazunari Sasaki 1 2 3 , Kengo Haga 2 , Tomoo Yoshizumi 2 , Daisuke MInematsu 2 , Eiji Yuki 2 , Run-Ru Liu 2 , Chie Uryu 1 , Toshihiro Oshima 2 , Teppei Ogura 3 , Yusuke Shiratori 2 , Kohei Ito 1 2 , Michihisa Koyama 3 , Katsumi Yokomoto 4
1 International Research Center for Hydrogen Energy, Kyushu University, Fukuoka Japan, 2 Faculty of Engineering, Department of Hydrogen Energy Systems, Kyushu University, Fukuoka Japan, 3 Inamori Frontier Research Center, Kyushu University, Fukuoka Japan, 4 Office for the Promotion of Safety and Health, Kyushu University, Fukuoka Japan
Show AbstractSolid Oxide Fuel Cells (SOFCs) are attractive energy systems for stationary, portable, and automotive applications. In realizing “fuel-flexible SOFCs”, various types of fuels may be applied directly or via a simple reforming process, including hydrocarbons, alcohols, biogas, coal gas, besides hydrogen. However, various types of minor constituents in practical fuels and/or from the system components can cause chemical degradation of SOFCs, such as anode and cathode poisoning phenomena, which can determine SOFC system life time. In this study, we therefore focus on the influence of external minor species, including sulfur, chlorine, phosphorus, boron, and siloxane for anodes, and H2O and SO2 for cathodes, on SOFC performance to have a general overview on long-term chemical durability of SOFCs. Chemical compatibility of Ni with foreign species has been thermochemically considered, by calculating equilibrium stability diagrams of C-H-O-Ni-X and Ni-H-O-X systems (X: external minor impurity elements). The stability of cell voltage, electrode overpotential, and IR loss up to 3000 hours has been experimentally examined to consider both equilibrium and kinetic phenomena, for H2-based fuels, for hydrocarbon-based fuels, and for simulated reformed fuels. Degradation rate and tolerant concentration for specific impurities have been obtained. Electrode microstructural changes and chemical compositions were also examined by various analytical methods. Chemical degradation mechanisms have been classified into several cases, including adsorption-type, sublimation-type, deposition-type, grain-growth-type, and eutectic-type, affecting long-term durability of SOFC systems.
12:30 PM - **GG11.8
Thermodynamic and Kinetic Measurements on Solid Oxide Fuel Cells.
Anil Virkar 1
1 Materials Science & Engineering, University of Utah, Salt Lake City, Utah, United States
Show AbstractTypical studies on solid oxide fuel cells are either on performance measurements on full cells or on half cells for polarization studies with the stipulation that half cell measurements can be used to predict full cell performance. The present work is on the measurement of relevant thermodynamic and kinetic measurements directly on thin electrolyte - anode-supported cells. Towards that end, anode-supported cells have been made using embedded platinum probes. Both DC and AC measurements are made - one set leads to the measurement of local thermodynamic parameters, one set leads to the measurement of kinetic parameters. It is proposed that two different types of equivalent circuits are needed to analyze the data. The paper will discuss what information measurements on embedded probes yield.
GG12: Diagnostic Techniques
Session Chairs
Thursday PM, December 02, 2010
Back Bay A (Sheraton)
2:30 PM - GG12.1
Atomic-scale Chemical Imaging of Core Shell-structured Pt3Co Fuel Cell Catalysts in an Aberration-corrected Scanning Transmission Electron Microscope.
Julia Mundy 1 , Huolin Xin 2 , Randi Cabezas 1 3 , Zhongyi Liu 4 , Junliang Zhang 4 , Nalini Subramanian 4 , Rohit Makharia 4 , Frederick Wagner 4 , David Muller 1
1 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 2 Department of Physics, Cornell University, Ithaca, New York, United States, 3 , Florida International University, Miami, Florida, United States, 4 Electrochemical Energy Research Lab, General Motors, Honeoye Falls, New York, United States
Show AbstractCompared to pure platinum (Pt) catalysts for use in proton exchange membrane fuel cells, Pt – based 3d metal (e.g. cobalt - Co) alloy catalysts have 2-4 times enhanced kinetic activity. They therefore, offer a practicable pathway to lower the Pt loadings and overcome the cost barrier for full-scale commercialization of fuel cell vehicles. Here, we investigated a commercial Pt3Co catalyst, with and without additional post-treatment, using a 5th-order aberration-corrected scanning transmission electron microscope (STEM) capable of both a high spatial resolution (~1Å) and a high usable collected beam current (~300 pA) for fast chemical imaging based on electron energy loss spectroscopy. Core/shell structures were observed. From the obtained atomic-scale chemical images of hundreds of Pt3Co nanoparticles, we statistically determined the Pt shell thickness as a function of specimen treatment, particle size, crystal surface orientation, and crystal order/disorder structure. We also saw evidence that both Ostwald ripening and coalescence contribute to the growth of the Pt3Co nanoparticles after fuel cell voltage cycling. The fundamental understanding gained from this work will be critical for the design and development of next-generation high-performing robust catalysts for fuel cell-powered vehicles with high energy efficiency and zero emissions.
2:45 PM - GG12.2
Atomic-Scale Chemical Imaging of Surface Ordering and Segregation in Pt-Co Electrocatalysts by Aberration-corrected Electron Microscopy.
Huolin Xin 1 , Julia Mundy 2 , Randi Cabezas 2 , Lena Kourkoutis 2 , David Muller 2 3 , Vic Liu 4 , Junliang Zhang 4 , Nalini Subramanian 4 , Rohit Makharia 4 , Frederick Wagner 4
1 Department of Physics, Cornell University, Ithaca, New York, United States, 2 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 3 Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York, United States, 4 Electrochemical Energy Research Laboratory, General Motors, Honeoye Falls, New York, United States
Show AbstractPt-Co nanoparticles are of particular interest in polymer electrolyte membrane fuel cells, as Pt3Co is one of the best catalysts for the reduction of oxygen. A direct determination and visualization of the arrangement of the two different species of atoms on different termination facets of Pt-Co nanoparticles is key to understanding the changes in electrocatalytic activity upon annealing, dealloying, and voltage cycling in a fuel cell. Aberration-corrected scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) is specially poised for this task as it enables rapid 2-D chemical mapping at atomic resolution. Using an EELS-optimized system, a 100kV, 5th-order aberration-corrected STEM, enabled us to collect more than one million EELS spectra providing maps of arrangement of Co and Pt atoms on the surfaces of hundreds of individual core-shell structured nanoparticles, both before and after fuel cell operations. The unprecedented quantity of data allows us to draw statistically significant conclusions about the chemical microstructure and correlations among microscopic degrees of freedom. To systematically study the impact of acid-leaching, annealing and voltage cycling on the Pt-Co nanocatalysts, we spectroscopically imaged ~80 particles as received from the commercial vendor (as-received), ~100 as-received particles that underwent accelerated fuel cell operations (voltage-cycled), ~60 as-received particles that were high-temperature annealed (annealed) and ~50 annealed particles that were subsequently acid leached (acid-leached). Our analysis of the Pt and Co chemical maps reveals the as-received particles have a 7 Å thick, relatively uniform Pt-rich shell surrounding the Pt-Co core. The voltage-cycled particles, on the other hand, demonstrate a significant divergence from the relatively homogenous as-received sample. In particular, the particles on average have grown in size and many particles exhibit multiple Pt-Co cores enclosed in a Pt-rich shell, indicating that particle coalescence is an important coarsening mechanism. There is also a strong dependence of the Pt-rich shell thickness on the particle size indicating that it is not only the loss of surface area, but also Co depletion that contributes to the loss of catalytic activity. The annealed particles show that the Pt-rich shell is removed. However, we find the preferential segregation of an atomically-thin Pt rich skin on the {111} but not {100} facet after high temperature annealing, the first such spectroscopic identification from an ensemble of nanoparticles. When these particles are subjected to an acid treatment, the thicker Pt-rich shell of the as-received sample is restored without facet dependence. The work at Cornell was supported as part of the Energy Materials Center at Cornell (EMC2), an Energy Frontier Research Center (DOE #DE-SC0001086). Facilities support by the Cornell Center for Materials Research (NSF DMR-0520404).
3:00 PM - GG12.3
3D FEM-model for the Reconstruction of a Porous SOFC Cathode.
Thomas Carraro 1 , Jochen Joos 2 , Andre Weber 2 , Ellen Ivers-Tiffee 2
1 Institute for Applied Mathematics, Heidelberg University, Heidelberg Germany, 2 Institute of Materials for Electrical and Electronic Engineering, Karlsruhe Institute of Technology, Karlsruhe Germany
Show AbstractThe performance of a solid oxide fuel cell (SOFC) is strongly affected by electrode polarization losses, which are related to the composition and the microstructure of the porous materials.A model that can decouple the effects associated to the geometrical microstructure and to the material properties can give a relevant improvement in the understanding of the underlying processes. Based on the method described in [1,2] a porous cathode can be reconstructed. Focused Ion Beam (FIB) and Scanning Electron Microscopy (SEM) techniques, combined with image processing, lead to an accurate reconstruction of the porous microstructure. We developed a detailed 3D finite element method (FEM) model for the calculation of the area specific resistance (ASR) as a performance index. In this model the electrochemical and diffusion processes are described by surface exchange parameters and diffusion coefficients. A parametric study of the main processes can reveal important influences on the performance index. In this context we perform a sensitivity study of the model with respect to the material parameters. The reconstruction of the microstructure allows to focus on the material parameters, considering the geometrical ones as accurate given data.Numerically, the solution of the forward problem is challenging due to the electrochemical couplings between the phases and the large dimension (bigger than 107 elements) of the model. Thus an efficient solver and a state-of-the-art optimization algorithm are key components of this work. We present a parameter estimation study, for a LSCF cathode, based on optimization methods for partial differential equations (PDE).[1] J. R. Wilson, W. Kobsiriphat, R. Mendoza, H. Y. Chen, J. M. Hiller, D. J. Miller, K. Thornton, P. W. Voorhees, S. B. Adler and S. A. Barnett, "Three-dimensional reconstruction of a solid-oxide fuel-cell anode", nature materials 5, pp. 541-544 (2006).[2]B. Rüger, J. Joos, T. Carraro, A. Weber and E. Ivers-Tiffée, "3D Electrode Microstructure Reconstruction and Modelling", ECS Trans. 25, pp. 1211-1220 (2009).
3:15 PM - GG12.4
STEM/EELS Characterization of Thin Film Solid Oxide Fuel Cell Cathodes with Improved Electrocatalytic Properties.
Donovan Leonard 1 , Yang Shao-Horn 2 , SungJin Ahn 2 , Ethan Crumlin 2 , Eva Mutoro 2 , Michael Biegalski 3 , Hans Christen 3 , Albina Borisevich 1
1 Mat. Sci. & Tech. Div, ORNL, Oak Ridge, Tennessee, United States, 2 Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Center for Nanophase Materials Sciences, ORNL, Oak Ridge, Tennessee, United States
Show AbstractHigh temperature solid oxide fuel cells (SOFCs) are a commercially viable electro-chemical alternative for distributed power applications. The cathode of a SOFC is responsible for electroreduction of dioxygen and subsequent transport of O2- to the electrolyte [1,2]. Impedance spectroscopy data from epitaxial perovskite thin film cathodes, with composition La0.8Sr0.2CoO3, showed improved electrocatalytic properties as well as higher oxygen non-stoichiometry when compared to bulk LSC[3]. The LSC113 thin films were grown on yttria stabilized zirconia (YSZ) and strontium titanate (STO) substrates by pulsed laser deposition. To better understand and improve the electrocatalytic properties of the ceramic thin films, the nano- and pico-scale mechanisms responsible for improved oxygen reduction were studied. Atomic resolution high angle annular dark field (HAADF) micrographs were acquired using a dedicated scanning transmission electron microscope (STEM), VG 501UX, operated at 100kV, equipped with a Nion aberration corrector and Gatan Enfina electron energy loss (EEL) spectrometer. EEL spectrum imaging was used to document chemical composition for regions of interest in the cathode thin film cross sections. The LSC thin film, Gd doped CeO2 buffer layer, and YSZ substrate were easily distinguished in the high angle annular dark field (HAADF) micrographs. For comparison, HAADF micrographs of the LSC/STO sample were also acquired. Contrast modulation, accompanied by unit cell distortion in the LSC thin films was observed in the samples with and without a buffer layer. This structure modulation, parallel to the interface, was present throughout the entire thickness of both the LSC/YSZ and LSC/STO thin films. In non-optimal growth conditions, second-phase inclusions were sometimes observed. The inclusions exhibited higher Co content compared to the LSC matrix, as detected by STEM/EELS. The structure modulation, parallel to the substrate interface and observed in both cathode thin film samples, may be evidence of oxygen vacancy ordering. The relationship of electrocatalytic properties of the thin film cathodes, lattice mismatch induced strain, and second phases will be discussed.This research is sponsored by the Division of Materials Sciences and Engineering and Scientific User Facilities Division, Office of Basic Energy Sciences of the U.S. DOE.References:[1] Gewirth, A.A. and Thorum, M.S., Inorg. Chem., 49 (2010) 3557.[2] Adler, S.B., Chem. Rev., 104 (2004) 4791.[3] G.J. la O et. al. Angewandte Chem., 49 (2010) 201001922.
3:30 PM - GG12.5
AFM-based Scanning Microwave Microscopy of SOFC-related Oxide Materials.
Alexander Tselev 1 , Evgheni Strelcov 2 , Keith Jones 3 , Roger Proksch 3 , Andrei Komakov 2 , Sergei Kalinin 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Physics Department, Southern Illinois University, Carbondale, Illinois, United States, 3 , Asylum Research, Santa Barbara, California, United States
Show AbstractDevelopment and optimization of new oxide materials for Solid Oxide Fuel Cells (SOFC) demand characterization tools capable to track processes occurring in this materials at various stages of the SOFC life with a nanometer-scale detail. In particular, alterations of the electrical conductivity, which are sensitive both to microstructure and composition of the complex oxide electrode materials, require simple and non-invasive techniques for local probing and characterization of the electrical properties at the nanoscale. Recently developed AFM-platform-based Scanning Near-Field Microwave Microscopy is an ideal tool for addressing this demand in the research of the SOFC materials. The principle of the technique is based on the dependence of the microwave impedance of the probe on the electrical properties of the material in the direct probe vicinity. It the system, a solid wire cantilever tip is connected to a coaxial cable (transmission line) through a matching network and then to a vector network analyzer. Amplitude and phase of the microwave signal reflected from the tip are measured while scanning over the sample surface in contact AFM mode. The system is realized on an Asylum Research MFP-3D AFM platform. The imaging can be performed in a frequency range from 1 GHz to 12 GHz. The advantage of microwave imaging over conducting AFM (c-AFM) is that it does not require fabrication of electrodes over the sample because the electrical circuit is complete through displacement currents between the AFM tip, sample, and outer conductor of the microwave line. The microwave signal carries information about conductivity of the materials directly under the tip apex. The very same tip is used for simultaneous contact AFM topography imaging. We demonstrate mapping of material conductivity with a resolution of about 20 nm, which is limited by the probe tip radius, using as model systems homogeneous bulk ceramic materials, thin films of complex oxides as well as heterophase domain structures appearing during metal-insulator transition in VO2 microcrystals (nanoplatelets). All these material systems are closely related to the oxides used in SOFCs, and the results clearly demonstrate a tremendous potential of the technique in this particular application. Research at ORNL's CNMS was sponsored by the Division of Scientific User Facilities, OBES, U.S. DOE. The research at SIUC was supported through NSF ECCS-0925837 and SISGR-DOE ERKCM67.
3:45 PM - GG12.6
Oxygen Reduction Kinetics at Pt | CsHSO4 by Conducting Atomic Force Microscopy.
Mary Louie 1 , Adrian Hightower 3 , Sossina Haile 2 1
1 Chemical Engineering, California Institute of Technology, Pasadena, California, United States, 3 Department of Engineering, Harvey Mudd College, Claremont, California, United States, 2 Materials Science, California Institute of Technology, Pasadena, California, United States
Show AbstractFuel cell performance is often limited by sluggish catalysis at the electrodes, yet the mechanisms for many electrochemical reactions remain elusive. Characterization of electrode kinetics is typically performed on electrodes of macroscale dimension and therefore provides an ensemble-averaged response. However, it is expected that nanoscale heterogeneity plays a large role in fuel cell electrode kinetics. In this work, atomic force microscopy, coupled with AC impedance spectroscopy and cyclic voltammetry, is employed to examine oxygen reduction kinetics at the nanoscale Pt | CsHSO4 interface at 150 °C in a humidified air atmosphere. Impedance spectra revealed that oxygen reduction at Pt | CsHSO4 was composed of two processes, one exponentially dependent on and the other largely independent of overpotential. Both processes displayed near-ideal capacitive behavior with minimal distributions in their relaxation times. This observation is consistent with measurement of a nanoscale interface for which spatial averaging effects are minimized and, furthermore, enables the separation of multiple processes that would normally be convoluted in macroscale electrode measurements. The overpotential-activated processes was interpreted as a charge transfer reaction and analyzed within the Butler-Volmer framework. Measurement of the current-voltage characteristics revealed that the charge transfer process varied notably across the electrolyte surface. Specifically, across six independent experiments, the exchange coefficient, α, ranged from 0.1 to 0.6, and the exchange current, i0, spanned five orders of magnitude. A pronounced counter-correlation between α and i0 was observed, suggesting that the extent to which the activation barrier for charge transfer decreases under bias depends on the initial magnitude of that barrier under open circuit conditions. Furthermore, the observation of such a correlation across multiple independent sets of data demonstrates the suitability of conducting atomic force microscopy for comprehensive studies of electrochemical reactions at electrolyte-metal-gas boundaries.
4:30 PM - GG12.7
A DFT-Based Analysis of Pathways for Oxygen Incorporation and Diffusion in Mixed Conducting Perovskites.
Eugene Heifets 1 2 , Yuri Mastrikov 1 3 , Rotraut Merkle 1 , Eugene Kotomin 1 2 , Joachim Maier 1
1 , Max-Planck-Institute for Solid State Research, Stuttgart Germany, 2 Institute for Solid State Physics, University of Latvia, Riga Latvia, 3 Materials Science and Engineering Department, University of Maryland, College Park, Maryland, United States
Show AbstractAn extensive set of DFT calculations on LaMnO3 bulk and slabs has been performed and used as a basis to identify both the most probable reaction mechanism for oxygen incorporation into (La,Sr)MnO3-δ cathode materials. Using ab initio thermodynamic analysis we found that MnO2 –terminated [001] surface is the most stable surface under fuel cell operation conditions (high temperature, high pO2, cubic unit cell) [1]. Simulations of oxygen adsorption show that O2- , O22-, and O- are adsorbed atop Mn and atomic (O- ) adsorption is favorable in comparison to the molecular one (O2- and/or O22-). The formation of these adsorbed species is quite exothermic and has to be considered as chemisorption. Still, due to electrostatic repulsion and the negative adsorption entropy at typical solid oxide fuel cell working conditions the levels of coverage of molecular oxygen adsorbates are low (in the few percent range). Under these conditions, a mechanism in which the encounter of O- with a surface oxygen vacancy at the surface exhibits the fastest rate and, therefore, is rate-determining. The variation of the reaction rate and preferred mechanism(s) with adsorbate and point defect concentrations is discussed [1]. Modeling of oxygen vacancies in the bulk and at the surface of LaMnO3 shows much lower formation energies of the vacancies at MnO2-terminated [001] surfaces (3.3 eV at MnO2 terminated [001] surface vs. 4.3 eV in the bulk) and suggests strong segregation of the vacancies towards this surface (which is not the case for LaO termination). The barriers for oxygen vacancy diffusion are much smaller at the surface (0.67 eV) than in the bulk (0.95 eV). The surface vacancy diffusion controls oxygen incorporation into cathode though vacancy approaching practically immobile adsorbed O atoms.
[1] Y. A. Mastrikov, R. Merkle, E. Heifets, E. A. Kotomin, and J. Maier, J. Phys. Chem. C 114, 3017 (2010).
4:45 PM - GG12.8
Molecular Modeling of Water Percolation and Proton Transport in Fuel Cell Membranes.
Nagesh Idupulapati 1 , Ram Devanathan 1 , Michel Dupuis 1
1 , PNNL, Richland, Washington, United States
Show AbstractModeling the complex chemical environment of polymer membranes used as electrolytes in fuel cells is a challenging task, because the processes that are driven by changes in hydration level and temperature span multiple length and time scales. We have used hierarchical multiscale modeling to understand charge transfer, proton hopping, water network percolation and membrane morphology in polymer membranes that are potential candidates for use in fuel cells. At the most fundamental level, our simulations employ density functional theory to study proton dissociation in model pendants that are building blocks for existing and proposed membranes, such as Nafion, sulfonated poly ether ether ketone and sulfonyl imides. At the next higher level, parameters extracted from density functional theory calculations have been implemented in a quantum hopping molecular dynamics study of proton hopping in Nafion membrane. At the highest scale simulated, we employ classical molecular dynamics simulations with various force fields to examine water networks, and the transport of water molecules and hydronium ions in Nafion, Hyflon, Dow, and sulfonyl imide membranes. The effect of hydration level and temperature on membrane nanostructure and molecular transport was systematically examined. In Nafion, water network percolation was found to occur above hydration level of 6 H2O per SO3-. In aromatic membranes, much higher hydration levels are required to achieve percolation. Our results for water diffusion coefficient, proton diffusion coefficient and radial distribution functions for various interacting species help interpret experimental observations. The superior transport properties of Nafion will be discussed in terms of acidity, backbone and sidechain flexibility and pathways for water networking. Our results provide molecular-level fundamental understanding of the desirable characteristics of proton exchange membranes. Such understanding can enable rational design of novel membranes for fuel cell applications.
5:00 PM - GG12.9
Measuring Oxygen Reduction Reaction in Fuel Cells on the Nanoscale.
Amit Kumar 1 , Stephen Jesse 1 , Sergei Kalinin 1
1 CNMS, Oak Ridge National Lab, Oak Ridge, Tennessee, United States
Show AbstractElectrochemical energy conversion systems based on gas-solid interactions such as solid oxide fuel cells (SOFC) and Li-air batteries are one of integral components of current and future energy technologies. The energy conversion in these systems is underpinned by a series of complex mechanisms like ion and vacancy diffusion, electronic transport and solid-gas and solid-liquid reactions at surfaces and triple phase junctions. One of the critical steps in the SOFC and Li-air battery operation leading to large overpotentials and charge-discharge hysteresis was the kinetics of the oxygen oxidation reaction (ORR). While it is well-recognized that ORR efficiency can be greatly enhanced by catalytic particles or morphologies, the mechanisms beyond this enhancement remain elusive, largely due to the lack of experimental techniques capable of probing ORR on the nanoscale. Here, we explore an alternative approach for probing of ion diffusivity and electrochemical reactivity on the nanometer length scales on the free electrochemically active surfaces and packaged devices, providing insight into the energy conversion and storage device operation on a level of single structural element with ~10 nm resolution. The Scanning Probe Microscopy (SPM) tip concentrates an electric field in a nanoscale volume of material, resulting in gas-solid or liquid-solid reactions, oxygen vacancy or proton injection, and ionic and vacancy diffusion. The vacancy movement and ion mobility results in localized strain under the tip which in turn can be probed by strain-bias coupling. We demonstrate local strain generation on the nanometer scale and correlate it directly to local oxygen ion diffusivity as well as the localized oxygen reduction for a LSC/GDC/YSZ system. These results are then compared to those obtained using classical characterization methods like electrochemical impedance spectroscopy which have already revealed markedly increased ORR activity up to two orders of magnitude on the epitaxial LSC surface in comparison to bulk, which may be attributed to increased oxygen vacancy concentrations in the films. The electrical field-dependence of ionic mobility is explored to determine the critical bias required for the onset of electrochemical transformation, potentially allowing to deconvolute reaction and diffusion processes in the fuel cell system on a local scale. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy.
5:15 PM - GG12.10
Diffusion Reactions and 3D-EBSD Analysis of Ni-YSZ Anode for Solid Oxide Fuel Cell.
Lax Saraf 1 , D. King 2 , A. Lea 1 , Z. Zhu 1 , J. Strohm 2 , D. Baer 1
1 EMSL, Pacific Northwest National Laboratory, Richland, Washington, United States, 2 Energy and Environmental Sciences Division, Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractUncontrolled electro-chemical reactions at Ni-YSZ solid oxide fuel cell (SOFC) anode in the hydrocarbon gas environment at elevated temperatures produce phase transformations resulting Ni/NiO diffusion in YSZ matrix. Oxidation-reduction kinetics of Ni/NiO in Ni-YSZ is known to affect hydrocarbon fuel dissociation at SOFC anode. The diffusion and phase transformation reactions occur at both atomic (nm) and bulk (µm) length scales. The 3D structural and pore space analysis is essential in SOFC anode to measure the extent of Ni migration, visualization of real-life pore interconnectivity and to calculate the triple phase boundary regions, crucial for hydrogen oxidation reaction. We discuss the pore space and crystal orientations in Ni-YSZ using 3D-electron backscatter diffraction (EBSD) and compare the data with dense yttria-stabilized zirconia (YSZ). The 3D-EBSD data is collected using successive slicing of Ni-YSZ plane using focused Ga-ion beam in a dual-beam microscope followed by EBSD structural grain orientation mapping on each slice. The stack of EBSD maps with known separation are reconstructed to get the volume information in the third dimension. We also discuss and model Ni/NiO diffusion reactions at both atomic as well as bulk length scales in Ni-YSZ as a result of hydrogen reduction followed by methane steam reforming. Surface analysis techniques such as Auger electron spectroscopy (AES), time-of-flight secondary ion mass spectrometry (ToF-SIMS) were also utilized to get a broader perspective of Ni/NiO diffusion reactions in YSZ matrix. These results provide a glimpse of complex reactions occurring in SOFC during internal reforming conditions.#Partial Reference- L.V. Saraf et al; Journal of The Electrochemical Society, 157 (4) B463-B469 (2010)
5:30 PM - GG12.11
Thermal Imaging of Solid Oxide Fuel Cells Operated with Alcohol Fuels.
Michael Pomfret 1 , Daniel Steinhurst 2 , Jeffrey Owrutsky 1
1 Chemistry Division, United States Naval Research Laboratory, Washington, District of Columbia, United States, 2 , Nova Research, Inc, Alexandria, Virginia, United States
Show AbstractA near-infrared (NIR) imaging system using a long wavelength filtered, silicon CCD camera has been developed to spatially and temporally map temperatures in real time on anodes of solid oxide fuel cells (SOFC) at temperatures exceeding 700 °C. This novel diagnostic technique is significantly cheaper, more sensitive, and easier to use than mid-IR imaging systems, providing a convenient way to characterize fuel cell components and processes. The approach has been used to determine thermal distributions and variations with < 0.1 mm spatial resolution and < 0.1 °C temperature resolution for anode-supported SOFCs operating on hydrocarbon and alcohol fuels. Anode processes and reactions, including fuel cracking, carbon deposition, and the oxidation of various species, are observed to induce temperature changes on the anode that reflect the chemical and material changes that occur as a function of operating conditions, such as fuel, temperature, and cell polarization. While the anode temperature increases when operating with hydrogen at OCV, it decreases when using hydrocarbon fuels due to endothermic carbon deposition. The temperature decreases more rapidly with propane than with methane, which indicates a higher propensity for cracking and carbon deposition with the heavier fuel. After operating with propane, there is more heating from electrochemical oxidation compared to methane, which indicates more carbon deposited. Current studies are aimed at investigating the effects of using methanol and ethanol as fuels in nickel/yttria-stabilized zirconia anode-supported SOFCs.Low-weight alcohols represent viable options for sustainable fuels in alternative energy systems, because they can be derived from agricultural byproducts, have relatively high energy densities, and have logistical advantages – with respect to their easier handling – over low-weight hydrocarbons. In SOFCs, an added benefit of alcohols is that they can undergo extensive direct, internal reforming under operational conditions. In cells operating at 800 °C, both methanol and ethanol fuel flows result in larger decreases in temperature (~-5 °C) than alkane fuels (~-3 °C), indicating more active endothermic fuel reactions – such as fuel cracking and carbon deposition. However, when the cells are operated under oxidizing conditions after exposure to the fuels, the cells exposed to ethanol show significant heating. Heating profiles in cells exposed to methanol resemble those of cells without carbon deposits. These results suggest that operation under methanol does not result in significant carbon deposition at 800 °C, while ethanol fuel flows lead to extensive carbon build up in the anode structure. This work signifies the first in-situ, real-time evidence of the different carbon reactions that occur with alcohol fuels. The results represent experimental confirmation of theoretical analysis of these two fuels in SOFCs.